J . Phys. Chem. 1991, 95, 7785-7791
7785
Interaction of CO with 0, N, and NO on Ru(001): Evldence for Bridge-Adsorbed CO, Site Changes, and Formation of Isocyanate Species K.L.Kostov: P. Jakob, H. Rauscher,
and D. Menzel*
Physik Department E 20, Technische Universitat Miinchen, W-8046 Garching, Federal Republic of Germany (Received: December 20, 1990)
In a search for the formation of interesting new molecular species, the interaction of CO with oxygen and dissociated NO on Ru(001) has been studied by using TPD, HREELS, and LEED. At 120 K, CO is asdorbed onto disordered or ordered (2 X 2) oxygen layers in a bridge form with the u(C0) vibration at 1850 cm-' not observed before, in addition to a linear form [u(CO) 2050 cm-I]. These two forms are observed also when CO is adsorbed onto partially or fully dissociated NO layers; some interesting interaction properties of molecular NO and CO, such as reciprocal site changes, emerge. After these N-containing coadsorbate layers are heated, a new loss peak at 2170 cm-' appears in the temperature region 200-350 K. This peak is interpreted as due to the formation of an isocyanate (NCO) surface species. The mechanism of its formation process is discussed; it appears to consist in a reaction between N adatoms and adsorbed CO, where the latter is most likely to be the bridge species.
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1. Introduction At well-defined conditions (pressure, temperature, preparation of the surface, etc.) some coadsorbate systems may exhibit surprising features including formation of new surface species. This is interesting in itself but also has consequences for practical applications. For example on supported transition-metal catalysts a surface isocyanate (Ni,'J R u , Rh,*IO ~ Pd,6 Ir,6J1 and R6J2-14), species is formed as a result of the reaction between N O and CO. This species is characterized in IR spectroscopy by two bands at 2260-2300 and 21 70-2200 cm-l. Initially, these two bands were attributed to a covalently bonded NCO species and to an anionic NCO- species bonded to a supported metal atom with a partially positive charge, re~pectively.~ Recent investigations suggest that the high-frequency band originates from NCO species bonded to the support s u r f a ~ e ; ' ~the J ~ N C O species characterized by the lower frequency band is now believed to be bonded to supported metal atoms.'*I0 Brown and Gonzales have proposed a detailed mechanism of N C O formation on supported Ru catalysts which includes the formation of an N O + 2CO ~ o m p l e x .Recently, ~ Davydov and Bell proposed a mechanism in which the first step is the production of N adatoms on the surface followed by a reaction between N and CO(gas).' A similar reaction mechanism has been proposed for supported Ir" and Pti2catalysts. After adsorption of HNCO on Rh( 1 1 I),'' Pt( 1 IO),'* and R(11l),I9 the adsorbed NCO groups dissociate to N adatoms and adsorbed CO at temperatures between 330 and 380 K. The authors of ref 19 suggest the isocyanate formation to proceed via a reaction of CO with adsorbed N atoms. In very recent work, Novak and Solymosi have adopted this mechanism for supported Rh catalysts.20 This mechanism is similar to the one believed to be operative in the formation of N 2 0 from Nad, and Noad, On w(1 lo).2' We did not find data in the literature for surface isocyanate formation on monocrystalline surfaces of the group VI11 transition metals as a result of CO and N O interaction. Apart from the work on supported catalysts mentioned above, there are such data for polycrystalline Pt film at about 300 K.22 Additionally, it has been pointed out that desorption of Nz from N-covered Cu( 1 1 1) a t high CO pressure (>O.l Pa) may occur via an isocyanate intermediate.23 Our HREELS measurements of dense layers consisting of molecularly coadsorbed N O and CO on Ru(001) formed at 1 15 K show that no new surface species forms during heating to 400 K.24 One of the reasons for this may be that dissociation of N O to N and 0 adatoms does not occur in such a layer in the range of stability of the N C O species. Therefore, we studied CO adsorption onto fully or partially dissociated layers 'Permanent address: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1040,Bulgaria.
of adsorbed N O on Ru(001) in a search for a path to such a new surface species. During heating of these complex coadsorbate layers to 350 K we indeed observed a new HREELS peak at 2170 cm-I, which corresponds well to the u(NC0) vibration. To exclude that this new species may result from an interaction between C O and the 0 atoms also present in fully or partially dissociated N O layers, we also investigated coadsorbed layers of CO with various oxygen layers. The negative result of these experiments with regard to the new species supports the assumption that the latter is due to a surface reaction between N and CO. While the main emphasis of this paper lies on the clear identification of an isocyanate species which can only form by reaction of CO with an N-containing species, the comparison to the 0 CO interaction is important in this context and has furthermore resulted in a new aspect compared to earlier work. Previous studies of CO 0 coadsorption on Ru(001) showed that in the presence of adsorbed oxygen the C O bond with the surface is weakened; for higher precoverages of oxygen, the desorption peak of CO is shifted down to 375 K, i.e., considerably below that of a pure, low-coverage CO The HREELS spectra of this coadsorbate system showed an upward shift of the u(C0) frequency to 2089 cm-I, consistent with a linear species with a weakened surface bond.25 In contrast to these results, we find
+
+
(1) Morrow, B. A.; Moran, L. E. J . Phys. Chem. 1977,81, 2667. (2)Morrow, B. A.; Sont, W. N.; St. Onge, A. J . Coral. 1980, 62, 304. (3)Davydov, A. A.; Bell, A. T. J . Carol. 1977, 49, 345. (4)Brown, M. F.;Gonzales, R. D. J . Caral. 1976, 44, 477. (5)Solymosi, F.; Rasko, J. J . Coral. 1977, 49, 240. (6) Unland, M. L. J . Carol. 1973, 31,459. (7) Solymosi, F.;Sarkany, J. Appl. Surf. Sci. 1979, 3, 68. (8) Arai, H.; Tominaga, H. J . Coral. 1976, 43, 131. (9)Hecker, W.C.; Bell, A. T. J . Catal. 1983,84, 200. (IO) Hecker, W. C.; Bell, A. T. 1984,85, 389,and references cited therein. ( 1 1 ) Solymosi, F.; Rasko, J. J . Caral. 1980, 63,217. (12)Unland, M.L. J . Phys. Chem. 1973, 77, 1952. (13) Solymosi, F.;Sarkany, J.; Schauer, A. J . Caml. 1977, 46, 297. (14)Lorimer, D.;Bell, A. T. J . Caral. 1979, 59, 223. (15)Dalla Betta, R. A.; Shelef, M. J . Mol. Caral. 1975, I , 431. (16)Solymosi, F.; Volgyesi, L.; Rasko, J. Z. Phys. Chem. 1980, I20,79. (17)(a) Kiss, J.; Solymosi, F. Surf. Sci. 1983, 135,243. (b) Solymosi, F.; Berko, A.; Tarnoczi, T. I. Appl. Surf. Sei. 1984, 18, 233. (18) Solymosi, F.;Kiss, J . Surf. Sci. 1981, 108, 641. (19)Gorte, R. J.; Schmidt, L. D.; Sexton, B. A. J . Card. 1981, 67,387. (20)Novak, E.;Solymosi, F. J . Carol. 1990, 125, 112. (21) Masel, R. I.; Umbach, E.; Fuggle, J. C.; Menzel, D. Surf. Sei. 1979, 79, 26. (22)Rasko, J.; Solymosi, F. J . Caral. 1981, 71, 219. (23)Balkenende, A. R.; Gijzeman, 0. L. J.; Geus, J. W. Appl. Surf. Sd. 1989, 37, 189. (24)Kostov. K.; Rauscher, H.; Jakob, P.; Menzel, D. Unpublished results. (25)Thomas, G. E.;Weinberg, W. H . J . Chem. Phys. 1979, 70, 954. (26)Lee, H.-I.; Praline, G.; White, J. M. Surf. Sci. 1980. 91, 581.
0022-365419 1 12095-7785%02.50/0 0 1991 American Chemical Society
7786 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Kostov et al.
the existence of bridge-adsorbed CO on oxygen-precovered Ru(OOl), in addition to this somewhat shifted linear species. We believe that this new bridged CO species plays a role in the isocyanate formation. Therefore, we start our account with these results and then concentrate on the conjectured isocyanate species.
2. Experimental Section The experiments were carried out in a UHV apparatus (base pressure 6 X 10-9 Pa) equipped with HREELS, TPD, LEED, and work function change (A@) measurement facilities, as described in previous paper^.*^^** The usable resolution of the electron spectrometer was about 50 cm-I, at a count rate of the elastically scattered electrons from a clean Ru(001) surface of 3 X lo5cps. The HREELS spectra were recorded in the direction of specular scattering. For experiments of thermal evolutions, all loss spectra were measured when the crystal had cooled to 120 K, after it had been heated to the designated temperature. Thus, the vibrational spectra shown are likely to correspond to the frozen-in state reached by the preceding heating. The TPD equipment (quadrupole mass spectrometer, Balzers QMS 112 with Feulner cup) has been described in detail in ref 29. The heating rate used was 2.5 K/s. The cleaning procedure of the crystal consisted of several heating cycles under oxygen pressure and subsequent annealing to 1570 K in UHV to remove the adsorbed oxygen. The surface order and cleanness were checked by LEED, HREELS, and A@ measurement during hydrogen adsorption on Ru(001) at 270 K.M The NO and CO exposures on the sample were carried out in front of a microcapillary array to ensure homogeneous coverages.
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3. Results and Interpretation 3.1. Adsorption of CO onto Oxygen-Covered Ru(001). An earlier study of Madey et al. shows that at 300 K the saturation oxygen coverage (0.5 ML; 1 ML corresponds to 1 adsorbate particle per Ru surface atom) on Ru(001) contains 19% less oxygen than for adsorption at 100 K.31a Therefore, in our experiments the saturated oxygen layer at 120 K is expected to correspond to 0.6 ML. Its HREELS spectrum is shown in Figure la. One intense u(Ru-0) loss peak is observed at 600 cm-’. After adsorption of CO onto this layer to saturation at 120 K, loss peaks at 2060 and 1850 cm-’ appear in the spectrum (Figure 1b). The first of these peaks is attributed to the u(C-0) stretch of linearly bonded C0.32933 The other peak (at 1850 cm-I) is most probably due to CO adsorbed in a bridging configuration.” It is interesting that such a species is never observed in pure CO layers on Ru(001),32+33 even in coverage ranges where geometrically the occupation of bridge sites is likely. This has been explained by assuming that in the case of Ru(001) the hybridization is more important than the site.” Obviously this behavior of linear bonding to an apparently structureless surface is lifted by preadsorbed oxygen atoms. Figure l e shows the thermal desorption spectrum (mass 28) of the layer corresponding to Figure 1 b. Two intense peaks are observed at 220 and 375 K. Compared to the pure CO layer (Figure 1 f), the low-temperature desorption peak that terminates at about 260 K is new. At this temperature the loss peak of bridge CO has almost disappeared (Figure IC) while the large peak at 2060 cm-’ stays roughly constant (the slight increase by about 10% could be due to some conversion accompanying desorption (27) 443. (28) (29) (30)
Barteau, M. A.; Broughton, J. Q.; Menzel, D. Surf. Sci. 1983, 133,
Jakob, P.; Cassuto, A,; Menzel, D. Surf.Sci. 1987, 187, 407. Feulner, P.; Menzel, D. J. Vac. Sci. Techno/. 1980, 17, 662. (a) Barteau, M. A.; Feulner, P.;Stengl, R.;Broughton, J. Q.; Menzel, D.J . Carol. 1985, 96, I . (b) Feulner, P.; Menzel, D. SurJ Sci. 1985, 154,
465. (31) (a) Madey. T. E.; Engelhardt, H. A.; Menzel, D. Surf. Sci. 1975,48, 304. (b) Pfnllr, H.; Held, G.; Lindroos, M.; Menzel, D. Surf. Sci. 1989, 220, 43. (32) Thomas, G. E.; Weinberg, W. H. J . Chem. Phys. 1979, 70, 1437. (33) Pfnllr. H.; Menzel, D.; Hoffmann, F. M.; Ortega, A,; Bradshaw, A. M. Surf. Sci. 1980. 93, 431. (34) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy; Academic Press: New York, 1982; p 285.
L61
A
zoo
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Temperature I K I
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11
,212
2050
I 319 0
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C) 260K
. bl
i1 “ i 0
500
1000 1500 Energy L o s s
2000
2500
( c m - l I
Figure 1. HREELS measurements of (a) oxygen-saturated Ru(001) (Tab = 120 K), followed by (b) CO expcsure to saturation at 120 K and (c, d) heating of the layer to the indicated temperatures. The thermal desorption spectra (mass 28) of layer b and of a pure saturated CO coverage are shown in (e) and (f)?respectively.
of the bridged species or a small change of the dynamic dipole moment of the linear species by removal of the other species and/or ordering caused by annealing of the layer). Therefore, we can interpret the TD peak a t 120 K as due to desorption of bridgeadsorbed CO. The high-temperature peak at 375 K characterizes the desorption of linearly bonded CO with u(C0) = 2050 cm-l. A comparison of parts e and f of Figure 1 (saturated pure CO layer; = 0.68 ML35) shows that the total amount of adsorbed CO for the layer of Figure 1b corresponds to 0.26 ML, or about 38% of the pure saturated CO layer. It is known that oxygen adsorption at 120 K does not lead to the formation of an ordered adsorbed layer, but only after heating of the saturated layer to about 350 K does an ordered (2 X 2) LEED pattern appear, which is interpreted as three degenerate (2 X 1) domain^.^' Here we also observe a disordered surface layer if CO and oxygen are coadsorbed at 120 K,but after heating to 350 K, just below the high-temperature desorption peak at 375 K, a (2 X 2) LEED pattern appears. The spots become sharper at higher temperatures. It is likely that this order is dominated by the (2 X 1) oxygen layer. Bridge and linear forms are observed also for CO adsorbed at 120 K onto an ordered oxygen layer which had been formed by heating of a disordered oxygen layer (Tab = 120 K) to 350 K. This can be seen clearly from HREELS measurements of an ordered oxygen coverage (9, = 0.6 at 120 K), saturated with CO a t 120 K. For this coadsorbed layer the thermal de(35) Pfnllr, H.;Feulner, P.;Menzel, D. J . Chem. Phys. 1983, 79, 4613, and work cited therein.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7787
Interaction of C O with 0, N, and N O on Ru(001)
295
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Temperature IKI
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a)
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1000 1500 2000 2500 E n e r g y Loss I c m - l I Figure 2. HREELS measurements of (a) CO adsorption at 120 K (to
0
500
saturation) onto an ordered oxygen layer, preprared after heating of adsorbed oxygen to 350 K (T,& = 120 K,eo = 0.680,,,) and (b, c) subsequent heating of the layer to the indicated temperatures. The thermal desorption spectrum (mass 28) of layer a is shown in (d). The coverages refer to adsorption at 120 K. sorption spectrum shows peaks at 295,380, and 435 K and some low-intensity structure at 190-230 K (Figure 2d). The HREELS spectra at several temperature points of this spectrum are shown in Figure 2a-c. Similarly to the results for the disordererd 0 layer (Figure 1 b), we observe loss peaks at 1850 and 2050 cm-' which characterize bridge-bonded and linearly bonded CO, respectively (Figure 2a). We measure almost the same vibrational spectrum at 250 K, before the thermal desorption peak at 295 K (Figure 2b). The loss peak of bridge-adsorbed CO at 1850 cm-' disappears after heating to 330 K but that of linearly adsorbed C O at 2050 cm-' remains (Figure 2c). Therefore, the peak at 295 K in Figure 2d can be attributed to desorption of bridge-adsorbed CO. Again similarly to Figure le, we observe high-temperature peaks of linearly bonded CO at 380 and 435 K; these two TPD peaks were observed also by Thomas and Weinberg for the same coadsorbate system.25 On the other hand, this early work did not report the loss a t 1850 cm-' in their HREELS results but only the linear species after CO adsorption at 1IO K onto an ordered oxygen layer. This may be due to their investigating only low and saturated oxygen precoverages, for which we find the amount of bridge CO to be zero and small, respectively. In the latter case we observe only a weak TPD peak of bridge C O at 290 K (similar to the corresponding one in Figure le) and a mere broadening of the loss peak at 2050 cm-I. The lower resolution in this early work ( 1 1-1 5 meV) may have prevented the observation of this broadening. The LEED measurements show an apparent (2 X 2) pattern for the ordered oxygen layer. After subsequent exposure to C O at 120 K and heating of the coadsorbate layer to 500 K (which removes all CO), the symmetry of the pattern is conserved. However, above 273 K the v(C0) loss intensity of linearly bonded CO increases by about 30%, which may be due to an ordering of the adsorbed CO molecules, since a similar effect of ordering on the loss intensity has been observed for pure C O layers.33
Additional HREELS measurements not shown herez4 lead us to the conclusion that the weak TPD structure at 190-230 K (Figure 2d) is connected to desorption of bridge-bonded CO and some linearly bonded CO. Comparison of the TPD spectra of Figures l e and 2d shows that even on the disordered oxygen layer a weak CO peak at -290 K exists. The corresponding strong TPD peak for C O in the ordered oxygen layer has been correlated with bridge CO, which apparently forms optimally only with ordered oxygen. This in turn makes it likely that small ordered oxygen domains are formed even if oxygen is adsorbed at 120 K. This is consistent with the broadening of the HREELS peak at 2050 cm-' in the region of the bridge CO frequency (- 1850 cm-I) just below its desorption at -290 K (Figure IC). At 323 K this broadening disappears (Figure Id). In agreement with Thomas and WeinbergZ5we observe a shift of the metal-CO stretch frequency to lower values (365 and 400 cm-I; Figures 1b and 2a) by 0 coadsorption which can be interpreted as due to weakening of the metal-CO bond by oxygen. As a summary of this section we note that C O adsorbs onto disordered and ordered oxygen layers in two forms: bridge and linear. The bridge species desorbs at rather low temperatures, so that it seems to be bonded to the surface quite weakly. In the initially disordered oxygen layer, a "linear" CO species is formed with properties (vibrational frequency and desorption temperature) close to those in the pure C O layer; on the ordered 0 layer a smaller amount of a more weakly bound linear species is found in addition. The details of the thermal desorption features of the bridge-adsorbed form depend on the oxygen ordering as well. This type of adsorbed C O desorbs in the case of an ordered oxygen layer at -295 K, but for the initially disordered oxygen layer the corresponding thermal desorption feature is shifted to lower temperature (-220 K). The highest loss frequency observed in any of these composite layers is at 2060 cm-I. 3.2. CO Adsorption onto a Partially Dissociated NO Layer. In this section we describe the HREELS results for the more complicated coadsorption system "CO plus partially dissociated NO" (Figures 3 and 4), where we have to envisage the possible interaction with 0, N, and NO. The mixed layers in the first series (A) of experiments (Figure 3) have been prepared as follows: (i) N O exposure of clean Ru(001) a t 115 K; (ii) heating of this coverage to 450 K to dissociate part of the adsorbed N O (Figure 3a) and prepare a layer consisting of adsorbed N O in v, (bridge) and v2 (linear) forms and of N and 0 ad atom^;^^ (iii) CO adsorption at 115 K onto this layer to saturation (Figure 3b); (iv) heating of the latter layer to the indicated temperatures (Figure 3c-f). At 453 K only v2-N0 and N and 0 adatoms remain on the surface, as shown by the HREELS spectrum (Figure 4a), in agreement with evidence from photoelectron s p e c t r o s ~ o p y . ~ ~ ~ * ~ ~ With this layer the second series (B) of experiments begins. The next steps in series B are repetitions of steps iii (Figure 4b) and iv (Figure 4c-f). Figure 3a shows the HREELS spectrum of NO/Ru(001) (9 = 0.8eN0,maxr adsorbed at 11 5 K) after heating to 450 K. We observe a low-intensity peak at 1490 cm-' due to vl-NO and a high-intensity peak at 1820 cm-' due to vz-N0.36937In agreement with previous work%we find that part of the vl-NO has dissociated to form 0 and N adatoms (losses at -550 cm-I). After exposure to CO at 115 K to saturation, the loss at 1820 cm-I broadens drastically at the high-frequency side (Figure 3b). No peak of linearly bonded CO can be separated. This is in contrast to CO adsorption on the clean Ru(001) surface, where adsorption of CO takes place exclusively in linear form with the C-0 stretch frequency at 1984-2061 cm-',32+33as well to that on oxygen layers (see section 3.1). We suggest that the broad loss consists of at least two peaks-one at 1825 cm-I for NO in the uZ state and another one at about 1850 cm-l from bridge-adsorbed CO. After
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(36) (a) Umbach, E.;Kulkarni, S.;Feulner, P.; Menzel, D. Sur/. Sci. 1979, 88,65. (b) Feulner, P.;Kulkarni, S.; Umbach, E.; Menzel, D. Sur/. Sci. 1980, 99, 489.
(37) Treichler, R.; Riedl, W.;Feulner, P.; Menzel, D. Sur/. Sei. 1991, 243. 239.
Kostov et al.
7788 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 366
A W
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Temperature I K I
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.
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500 1000 1500 2000 (cm-1) Figure 3. HREELS spectra (A series), recorded after (a) heating of a NO layer (eNO = 0.8eN0,mrx, Tab= 115 K) to 450 K, (b) subsequent CO exposure to saturation at 1 1 5 K, and (c-f) heating of the layer of (b) to the indicated temperatures. The thermal desorption spectrum (mass 28) of layer b is shown in (g). this layer is heated to 173 K, a peak at 201 5 cm-l from linearly bonded CO grows in and reaches a maximum at -270 K (Figure 3c,d). Parallel to this process, narrowing of the peak at -1820 em-l caused by elimination of the broadening at the high-frequency side is observed. Therefore, we believe that a transition of bridge-adsorbed CO to a linear form occurs. The TPD spectrum of the composite layer of Figure 3b shows no significant CO desorption up to -270 K (Figure 3g). Therefore, the conversion of bridge to linear CO is almost stoichiometric in this region. Also, overlapping TPD peaks are observed at 290 and 310 K. The first peak may correlate to a desorption of bridge CO adsorbed within ordered oxygen domains (see Figure 2 and section 3.1). The nature of the TPD peak at 310 K is not clear. Around 270 K we clearly observe the appearance of a new peak at 2170 cm-'. which reaches maximum intensity at -328 K (Figure 3d,e). The new surface species characterized by this vibration is observed only if CO is adsorbed on a partially (or fully; see below) dissociated NO layer on Ru(001). Above 328 K the intensity of the peak at 2170 cm-I decreases and disappears at 373 K (Figure 3f), which can be due to either dissociation or desorption of the new species. In this temperature region there is a peak at 353 K in the TPD spectrum of C O (Figure 3g). However, this peak is even more intense at lower, partially dissociated NO coverage where the new EELS peak at 2170 cm-' is significantly weaker.24 Therefore, we cannot connect this TPD peak with dissociation of the new surface species. Rather, we attribute this peak to the desorption of linear CO. After adsorption of CO, a low-frequency peak at 285 cm-I appears (Figure 3b) which can be attributed to the metal-CO stretch of bridged CO. During heating of the layer, the intensity of this loss decreases and a peak at 390 cm-' due to the metal-CO stretch of linear C O grows in (Figure 3). This parallels the conversion of bridge to linear C O as seen by the disappearance and appearance, respectively, of their u(C0) losses. As for the C O + 0 layers, a substantial weakening of the metal-CO bond
~
l
~
0
E n e r g y Loss
0
500 1000 1500 2000 E n e r g y L o s s Icm-1)
Figure 4. HREELS spectra (B series), recorded after (a) heating of the layer in Figure 3f to 453 K, (b) subsequent CO exposure to saturation at I15 K, and (c-f) heating of the layer of (b) to the indicated temperatures. The scale factors refer to the elastic peak in Figure 3a. The thermal desorption spectrum (mass 28) of layer b is shown in (g).
relative to the pure C O layer is seen as a result of N + 0 coadsorption. The appearance of linearly bonded CO [u(CO) = 2015 em-'] with increasing temperature is accompanied by an increase of the intensity of the peak at 1490 em-' which characterizes (bridged) vl-NO. Therefore, the transition of bridge CO [v(CO) = 1850 cm-'1 to the linear state [u(CO) = 2040 cm-I] is coupled with the conversion of linear v2-N0 to bridged vl-NO. Both processes are completed around 273 K. This correlation may be due to an optimization of the total surface coverage, but electronic effects may also play a role. Above 300 K, in parallel to the decrease of the loss features at 2030 and 2170 cm-l, a backtransformation of v,-NO to v2-NO occurs. This conversion is also observed in pure N O layers on R U ( O O ~ ) . ~ At ~ , ~-400 * K desorption of mass 30 (NO) starts and the peak of v,-NO in the HREELS spectrum has vanished at about 453 K, leaving only one intense peak at 1820 em-', characteristic of v2-N0 (Figure 4a). Its loss intensity is slightly higher (X1.2) than the corresponding one for the initial layer (Figure 3a). This may arise from the fact that besides some dissociation and desorption of adsorbed V,-NO'~the conversion to v,-NO occurs around 450 K,but again a change of cross section cannot be excluded. In addition to v2-N0 [u(NO)= 1820 em-'], the layer in Figure 4a contains 0 and N adatoms [u(Ru-0) and u(Ru-N) at about 590 cm-'1. However, the peak intensity of the dissociation products N and 0 is now significantly higher (about X2) than found in Figure 3a, and its maximum is shifted to higher frequency (590 cm-') (Figure 4a). This is probably a coverage effect, since the shift of u(Ru-0) of 0 on Ru(001) to higher values with increasing
I.
(38) Conrad, H.;Scala, R.; Stenzcl, W.;Unwin, R.Sur/. Sci. 1984,145,
Interaction of C O with 0, N, and NO on Ru(001) coverage is well establishedz and may also hold for N on Ru(001). Our thermal desorption measurements also show that up to 450 K there is negligible desorption of N adatoms as N2,in agreement with previous studies.36b From all this we conclude that in the case of Figure 4a there is a higher amount of N and 0 adatoms on the surface than for the experiments presented in Figure 3. Subsequent CO exposure of the layer in Figure 4a to saturation ( T = 1 15 K) again leads to a significant broadening of the loss peak of v,-NO at 1820 cm-l (Figure 4b). In contrast to Figure 3b we now also find a peak of linearly bonded CO at 2050 cm-I. This must be an effect of additional free sites in the second series (B) of experiments (Figure 4), as confirmed by the larger area of the CO TPD spectrum and the stronger peak of linear C O at 366 K (Figure 4g) compared to Figure 3g. After heating to 198 K, the loss peak of v,-NO at 1490 cm-' appears and-similarly to series A-its intensity increases with the surface temperature up to 273 K and then vanishes at about 373 K (Figure &f). We discussed the reason of this behavior above. In contrast to series A, now the new peak at 2170 cm-I appears at an even lower temperature (198 K) (Figure 4c). The maximum intensity of the new peak, attained at 333 K (Figure 4e), is about twice that of linear CO. It should also be mentioned that a peak of very low intensity appears at about 1330 cm-' when the new peak at 2170 cm-' is intense (Figure 4d,e); this is discussed in section 4.1. LEED measurements show a well-ordered (2 X 2) pattern during the experiments of series A. The same pattern is observed for series B, but the brightness of the spots is weaker. Thermal desorption spectra (mass 28) of the coadsorbate layers in Figures 3b and 4b show a stronger peak at 220 K in the case of series B (Figure 4g). In agreement with section 3.1 we interpret this peak as being due to desorption of bridge CO, adsorbed onto disordered oxygen domains. This is corroborated by the LEED data for the better ordered layer in series A mentioned above. As evident from Figures 3 and 4 the formation process of the new species is more obvious in the B series of experiments. For this series the amount of N and 0 adatoms is larger than the corresponding one for series A. Also, the new surface species was not observed for the C O + 0 layer (section 3.1), which excludes an interaction of 0 with CO as the reason for the new species. It is very probable, therefore, that adsorbed C O interacts with N adatoms to form the new species, but the conditions for this process are not clear yet. Also, we cannot exclude from these results a role of adsorbed v2-N0 molecules for this process in an almost dense layer on the Ru(001) surface. To study this, we carried out experiments dealing with postadsorption of CO onto a fully dissociated N O layer on Ru(001). 3.3. CO Adsorption onto a Fully Dissociated NO Layer. The experimental data (TDS and HREELS) for CO adsorption onto fully dissociated N O layers exhibit only minor changes with increasing NO precoverages above 0.5 ONOJnax ( Tab= 1 15 K). From earlier it is well-known that during the heating of higher NO coverages on clean Ru(001) not only dissociation of NO occurs but also molecular desorption can be observed. Therefore, the total amount of N and 0 atoms obtained by heating different precoverages of N O to 500 K is essentially independent of precoverage above half-~aturation.~~ After this heating, the residual NO layer is fully dissociated. The initial N O layer on Ru(001) ( O N 0 = 0.80N0,,ax adsorbed at 115 K) consists of NO molecules in v2, vI, and vo states with vibrational peaks at 18IO, 1490, and 1425 cm-l, respectively3* (Figure Sa). Heating to 500 K leads to a complete dissociation of adsorbed NO, as expected.36 The dissociation products, adsorbed N and 0 atoms, show peaks at about 575 cm-' (Figure 5b). Exposure of this layer to C O to saturation at 115 K leads to the formation of two peaks at 1850 and 2065 cm-l for bridge-adsorbed and linearly adsorbed CO, respectively (Figure 5c). In Figure 5g the thermal desorption spectrum (mass 28) of this layer is shown. Thermal desorption of masses 30 (NO), 26 (CN), and 42 (NCO) is not observed. The maxima of the first two TPD peaks of CO at -220 and -290 K coincide with those for the
-
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7789
370
L?
100 ,
/
Loo
200
300
LOO . , - 2050 500
1
91
TemperoturelKI
I\ U V L a) 0
115 K
1000 1500 2000 2500 Energy L o s s (cm-11 Figure 5. HREELS spectra recorded after (a) NO adsorption at 115 K (eNO= O.8BNomX): (b) subsequent heating to 5 0 0 K,(c) CO adsorption at I15 K to saturation onto layer b, and (d-f) subsequent heating to the 500
indicated temperatures. The thermal desorption spectrum (mass 28) of layer c is shown in (g). bridge CO in the cases of C O adsorption onto disordered and ordered oxygen layers, respectively (see section 3.1 and Figures l e and 2d). Also, the HREELS spectra exhibit a loss peak of bridge-adsorbed CO at 1850 cm-l (Figure 5c,d). Therefore, we interpret both thermal desorption peaks at -220 and -290 K in Figure 5g as due to desorption of bridge CO from disordered and ordered oxygen domains. Actually, after heating of the layer in Figure Sa to 500 K, very weak spots of a (2 X 2) LEED pattern were observed, which we attribute to ordered oxygen domains on the surface. This is in agreement with the LEED results from an earlier where the (2 X 2) pattern (correlated with an v2-N0 superstructure) disappeared at -400 K, but subsequent heating led again to an increase of the intensity of the (2 X 2) spots which was attributed to the remaining oxygen coverage. The TPD peaks at 370 and 418 K must originate from desorption of linear CO ( Y = 2060 cm-I), which is the only C O species present at these temperatures. This clear lowering of peak temperatures with respect to the pure CO layer on R U ( O O ~ indicates )~~ a substantial influence of coadsorbed N and 0 atoms on the C O desorption kinetics, as is the case for 0 atoms alone. In the simplest approximation, this is due to a decrease of molecular binding energy, in agreement with the evidence from vibrational frequencies, but in the absence of a detailed kinetic analysis no exact statement is possible. The broad peak around -530 K characterizes desorption of N 2 (from recombination of adsorbed N atoms),36 as also checked by following masses 12 and 14. Heating of the layer in Figure 5c to about 223 K leads to the appearance of the new loss feature at -2170 cm-I. This peak reaches its maximum at 273 K (Figure 5d) and disappears at about 350 K (Figure 5f). After this layer was cooled from 350 to 1 15 K and repeatedly heated to 300 K, the peak at 2170 cm-' did not reappear. Also, heating of the layer of Figure Sf to 500 K to remove part of the N atoms, followed by CO re-exposure at 1 15 K, leads to a similar HREELS spectrum as in Figure 5c. The
7790 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
loss peak of the new species at 2170 cm-l appears again after subsequent heating to 300 K. Its intensity is lower than in the case of Figure 5d. This correlates with the lower amount of adsorbed N compared to the initial layer of Figure 5b. There is no TPD feature that can be directly correlated with the disappearance of the vibrational loss of the new species. We conclude that the disintegration of the corresponding species is not desorption-limited. We interpret the small peak at 1800 cm-' in Figure 5f as adsorbed N O in the v2 state, but the reason for its presence on the surface is not clear (readsorption from the gas phase; recombination of N and O?). Similarly to the observation in Figure 4d, some HREELS spectra with strong peaks at 2170 cm-' contain a weak peak at 1330-1 340 cm-I (e.g., Figure 5d). In the same figure another low-intensity peak at 1500 cm-' is observed, which we attribute to a small amount of v,-NO resulting from conversion of v2-NO during CO adsorption.
-
4. Discussion 4.1. The Nature of the New Species. Our experimental results show clearly that the adsorbed C O participates in the process of formation of the new surface species which is characterized by the HREELS peak at 2170 cm-I. There are several possibilities for this surface reaction of adsorbed CO: (1) interaction with adsorbed N O (v2 form); (ii) interaction with oxygen adatoms; and (iii) interaction with nitrogen adatoms. i. The first possibility can be excluded because the experiments represented in Figure 4 show that in the case of series B with a comparable amount of v2-NO a much more intense peak at 2170 cm-' results than in series A (Figure 3) and because the new species also forms in a layer without v2-NO (section 3.3). Furthermore, experiments show that at various coverages of molecularly adsorbed NO at 1 15 K and subsequent CO exposure, the new surface species does not form after the coadsorbate layer is heated. 24 ii. The second possibility can also be excluded. Our results have shown that CO adsorbs onto oxygen-covered Ru(001) at 120 K in two forms: linearly bonded and bridge-bonded CO. The latter form of adsorbed C O desorbs at about 220 K in the case of an unordered oxygen layer and a t about 290 K for the preordered oxygen layer. However, a peak at 2170 cm-I was never observed when these mixed layers were heated. In previous work on adsorption of C O on silica- or aluminasupported ruthenium, three IR bands at 2030-2040 [u(CO) of CO adsorbed on reduced Ru], 2070-2080, and 21 30-2150 cm-l were observed.3w2 Initially, the two latter bands were assigned to u(C0) of CO adsorbed on a Ru atom perturbed by a nearby oxygen atom and to u(C0) of CO adsorbed on an unoxidized Ru surface, respe~tively.~~ Other authors attributed these bands to multicarbonyl species bonded to positively charged Ru atoms.4w2 In recent work, Yokomizo et a!3l assign both high-frequency bands to R U ~ + ( C Ospecies )~ bonded directly to the support by Si-0R u ( C O ) ~bonds. However, these interpretations cannot be correct in our case since under our experimental conditions it is not possible to oxidize Ru(001) and no loss with a frequency above 2060 cm-I has been observed for C O on the oxygen-covered surface; also, the frequency (2 170 cm-I) of our new loss feature appears to be too high for u(C0) of adsorbed CO only perturbed by oxygen adatoms. On the other hand, the interaction of CO with preadsorbed N O on silica-supported Ru does lead to the appearance of a new peak at 21 80 cm-I, interpreted as due to an isocyanate (NCO) specie^.^^^ iii. The adsorbed N atoms are electronegative (like C O molecules). Therefore, repulsive forces exist between CO and N on the surface and a shift of u(C0) to higher frequencies would (39) Brown, M. F.; Gonzales, R. D. J . Phys. Chem. 1976.80, 1731. (40) Davydov, A. A.; Bell, A. T. J . Catal. 1977, 49, 332. (41) Chen, H. W.; Zhong, Z.; White, J. M. J . Coral. 1984, 90, 119. (42) Solymosi, F.; Rasko, J. J . Catal. 1989, 115, 107. (43) (a) Yokomizo, G. H.; Louis, C.; Bell, A. T. J . Carol. 1989, 120, I . (b) Yokomizo, G. H.; Louis, C.: Bell, A. T. J. Coral. 1989, 120, 15.
Kostov et al. TABLE I: Vibrational Bands of Isocyanate, Formed as a Result of the Interaction between CO and NO on hpported Traesition-Met.l Catalysts and Polycrysuline R Film (IR) a d after Adsorption of HNCO on Pt(ll1) (HREELS) sample Ni/Si02 Pd/AI,O, Ru/Si02 R~/A1203 Rh/Si02 or AI2O3 Ir/AI2O3 Pt/Si02 Pt/AI,O, Pt film Pt(ll1) Ru(O0 1)
v,(NCO), cm-l 21 90-2201 2264 2180 2148-2238 2259-2267 2170-2190 2240-23 10 2239-2267 2190 2280 2148 2267 2180 2160 2170
ref 1, 2 6 3, 4 5, 6
9, 10, 20 6, 11 14 6, 12 22 19 this work
be a possible result, as for adsorbed 0. However, since coadsorbed 0 does not induce the new loss [although it does increase u(CO)], it is unlikely that coadsorbed N would; the expected shift from mere interaction would be smaller. So the remaining possibility is the formation of a new surface species by reaction between N and CO. As mentioned under Introduction, the formation of isocyanate (NCO) has been observed on many supported transition-metal catalysts. This species is also assumed to form on polycrystalline Pt films during N O and CO interaction around 300 K.22 Table I shows the IR band of isocyanate formed on supported catalysts as result of a reaction between CO and NO. Two bands are observed at approximately 2260-2300 and 2170-2200 cm-I. The first band has been attributed to N C O species bonded to the support surface and the lower frequency to species bonded to supported metal atoms.I0 This is corroborated by a comparison with data for transition-metal isocyanate complexes in which the NCO group is bonded to one metal atom through the nitrogen.@ The close agreement of the loss energy of our new species with the data mentioned in Table I, together with the exclusion of possible other explanations (i, ii), leads us to the conclusion that an NCO species, with similar bonding, forms on Ru(OO1) as well. Further support for this interpretation can be found in the published investigations of HNCO adsorption. On polycrystalline surfaces of Ru, Rh, Pd, Ir, and Pt, adsorbed HNCO dissociates to form N C O with u,(NCO) frequencies at 2176-2183 cm-1.22 Also, after fragmentation of HNCO on Pt(l1 I), a weak peak at 2160 cm-' was observed in the HREELS spectr~m,'~ which is most probably due to surface NCO. This species is unstable at higher temperatures and decomposes a t 370 K. Thus, the thermal evolution of the 2160-cm-l species on Pt( 1 1 1) is very similar to our observation on Ru(OO1). The u,(NCO) and u,(NCO) frequencies of gas-phase HNCO are at 2274 and 1327 cm-I, respe~tively!~ Also, there are bending modes at 572,670, and 797 cm-'. We do not observe peaks that could be assigned to bending modes of adsorbed NCO. Possible reasons are low intensities and the overlap with the u(Ru-0) and u(Ru-N) peaks around 600 cm-I. As to the us loss, in some of the HREELS spectra with the intense peak of u,(NCO) at 2170 cm-I, a peak of very low intensity a t 1330 cm-' was observed (Figures 4d,e and Sd), in good agreement with the u,(NCO) frequency of isocyanate complexes of transition metals44 and u,(NCO) of gas-phase HNCO which have a linear configuration. Because of this good agreement, it is very likely that the isocyanate species is bound to the surface in an on-top configuration. From the point of view of mode frequencies a surface isonitrile species (Ru-N=C) should also be considered. This possible explanation has already been discussed by Brown and Gonzales! They exclude the formation of a C z N group on the Ru/SiO, (44) Nakamoto, K. In Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley & Sons: London, 1986; pp 288-289. (45) Reid, C . J . Chem. Phys. 1950, 18, 1544.
Interaction of CO with 0, N, and N O on Ru(001) catalyst because no stable Ru compounds with a terminal isonitrile have been isolated.46 In addition to this argument, such a species is very improbable since dissociation of CO would be required for it, which is very unlikely on Ru(001). As a result of the discussion in this part we suggest that in our experiments an adsorbed NCO species is formed on Ru(001). Most probably this species is linearly bonded through its N atom to one Ru surface atom. 4.2. Formation of the Isocyanate Species. While overall this species must form from Nads+ toad,, the detailed mechanism is not clear. It is interesting that the formation only works under rather restricted conditions and that under these conditions various CO species exist on the surface. This poses the question if one of these species is uniquely suited for this reaction. If NCOad,forms from Nadsand COads,then the N atom has to be inserted into the bond between CO and the surface. This is reminiscent of the Hofmann rearrangement in organic chemi ~ t r y . ~ ' Certainly, the two reaction partners have to occupy adjacent surface sites before reaction; furthermore, the transition state is likely to contain a weakened, but still existing, Ru-CO bond and a direct interaction between N and C. Naively, one may expect that the bridge-bonded CO species would be best suited for such a process, since one could envisage it to untie one of its "legs" from the surface to newly engage it with the N atom, keeping the other down first. It appears worthwhile, therefore, to examine the conditions under which isocyanate formation appears to be easiest, to thereby check for a connection to the various CO species present. The experiments with repeated CO adsorption onto partially dissociated NO layers (series B of section 3.2) showed that an increased amount of N adatoms was correlated with an increase of bridge-adsorbed CO in the lower temperature state (desorbing at 220 K). Under these conditions the new loss peak at 2170 cm-' appeared at lower temperature (at -200 K) and reached a higher intensity than in series A. The larger amount of isocyanate could simply be due to the larger number of N atoms; however, the increased ease of formation with larger amounts of bridge CO present may suggest that this species is indeed involved in the reaction. One might argue against this connection that the lowest TPD peak of bridge CO occurs at 220 K, Le., considerably lower than the appearance of the isocyanate species in series A (270 K). However, there is also the 290 K peak of bridge CO; furthermore, since there is obviously interconversion between linear and bridged CO in the relevant temperature range, it could be envisaged that the bridged species is connected with the transition state, even though it is not stable any more at zero pressure and the same temperature. On the other hand, there might also be an influence of order. Series B, with the easier and more plentiful reaction, showed very little order compared to series A; order appeared to strengthen the 290 K TPD peak, so that an anticorrelation results in this respect. There may well be an effect of island distributions and/or of well-defined surroundings of the N adatoms. The situation is further complicated by the existence of the third type of CO, the linear species formed from conversion of bridged CO, which may behave differently. We do call attention to the result of the "recooling" experiment of Figure 5f which showed that N adatoms and normal linear CO do not react to form isocyanate. So while it is not possible at present to decide unequivocally whether the bridged CO species is indeed the one necessary for the N CO surface reaction, there are good arguments for this assumption. The N adatoms certainly occupy threefold sites, while the final NCO species very probably is adsorbed on top of a Ru atom. Hence, it is probable that the modification of N adatoms which is necessary to make the reaction possible consists in a shift of Nadsfrom the threefold to an on-top site: this shift must be i n d u d by the interaction with neighboring CO. One may further speculate that the R u atom corresponding to this on-top site is
+
(46) Clarke, R. E.; Ford, P. C. Inorg. Chem. 1970, 9, 227.
(47) Streitwieser, A., Jr.; Heathcock, C. H. Organische Chemie; Verlag Chemie: Weinheim, 1986; p 590.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7791 one of the two bridging Ru atoms of the initial bridge CO. The quasi-Hofmann insertion would then look like 0 0
I
/"\ /"\flu
Ru-Ru
:
-
-
0 'c-N
I
Au-Ru
I
: RU
I I N I
C Ru-RU
: RU
(where the : indicates the threefold site). This proposal is admittedly quite speculative, but it is in agreement with all evidence and can serve as a starting point for further investigations. In particular, more detailed information about isocyanate species itself, in particular its geometry, would be very important. For N 2 0 formation from N + N O on Pt(l1 l ) , a quite similar mechanism-albeit enhanced by a high electrostatic field-has very recently been supported by detailed quantum chemical calculation^.^^ Similar zero-field calculations for our case would be very illuminative and are planned. To be sure, quite a number of questions remain open, even if this mechanism is essentially correct. For instance, while the adsorbed oxygen atoms present in the dissociated NO layers have been shown not to be responsible themselves for the new species, they may well influence the reaction-probably in a negative sense. To test this, investigations using layers containing no 0 atoms besides N adatoms are planned. In this way, it is also hoped that a purer isocyanate layer can be prepared which then can be investigated with respect to the adsorption site and orientation by use of methods such as NEXAES and ARUPS. This is planned for the future. 5. Conclusions It has been found that CO adsorbed onto oxygen-covered Ru(001) at 120 K forms not only a linear species as reported earlier but also bridged adsorbates [v(CO) = 1850 cm-I]. The thermal desorption features of this bridge-adsorbed CO species depend on the oxygen ordering. In the case of an initially unordered oxygen layer it desorbs at -220 K, but from an ordered oxygen layer the corresponding thermal desorption feature is shifted to -290 K. The highest vibrational frequency observed for any 0 CO layer is 2060 cm-I. The same forms of CO are observed after adsorption at 1 15 K onto dissociative NO layers. For the codsorbate layer consisting of CO and partially dissociated NO (initial coverage 80% of saturation), the conversion of bridge-adsorbed CO to a linearly bonded form in parallel to transformation of (linear) v2-NO to (bridged) v,-NO occur during heating to -273 K. In the same temperature range we also observe the appearance of a new surface species with a loss peak at 2170 cm-l, which reaches maximum intensity at -328 K; a weak loss at -1340 cm-' was observed for conditions with well-developed 21 70-cm-l loss. The formation of the new species is more efficient when larger amounts of N and 0 adatoms are present and also takes place on fully dissociated NO layers. Our results and the comparison with transition-metal isocyanate complexes as well as the data for isocyanate formation on supported transition-metal catalysts suggest that these observations indicate the formation of surface isocyanate (NCO) species on Ru(001) by reaction between adsorbed CO and N adatoms. This species is very probably bound at an on-top site. It is likely that bridged CO is important for this reaction as reactant or transition state.
+
Acknowledgment. We appreciate a critical reading of the manuscript by H.-P. Steinruck. We thank the Alexander von Humboldt-Stiftung for a stipend to K.L.K. which made this work possible. This work has been supported financially by the Deutsche Forschungsgemeinschaft through SFB 338 and by the Fonds der Chemischen Industrie. Registry No. Ru, 7440-18-8; 02,7782-44-7; NO, 10102-43-9; CO,
630-08-0. (48) Kreuzer, H . J.; Wang, L. C. J . Chem. Phys. 1990, 93.6065.
