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Effects of preadsorbed oxygen on the formation and decomposition of NCO on Rh(111) surfaces
Applications of Surface Science 18 (1984) 233-245 North-Holland, Amsterdam
233
EFFECTS OF PREADSORBED OXYGEN ON THE FORMATION DECOMPOSITION OF NC0 ON Rh’(111) SURFACES F. SOLYMOSI,
A. BERKb
AND
and T.I. TARNCXZI
Reaction Kinetics Research Group, The University, P.O. Box 105, H -6701 Szeged, Hungary Received 26 January 1984; accepted for publication 8 May 1984
The interaction of HNCO with oxygen dosed Rh(ll1) surface has been investigated by Auger electron, electron energy loss and thermal desorption spectroscopy. The presence of adsorbed oxygen exerted no apparent influence on the weakly adsorbed HNCO (Tp = 130 K). It promoted, however, the dissociative adsorption of HNCO by forming a strong O-H bond which prevented the associative desorption of HNCO. As a result no H, and NH, formation occurred, in contrast with the clean surface, and the surface concentration of irreversibly bonded NC0 was also increased. New products of the surface reaction were Ha0 and CO,. in addition to CO and N, observed on a clean surface. From the behavior of the losses characteristic for the adsorbed NC0 it appeared that the preadsorbed oxygen exerted a significant stabilizing effect on the NC0 bonded to the Rh.
1.
Introduction
The behavior of HNCO on clean metals has been investigated so far on Cu(ll1) [1,2], Pt(ll1) [3], Pt(ll0) [4] and Rh(ll1) [5] surfaces. The primary motivation of this work was to provide data for a better understanding of the surface chemistry of NC0 intermediates formed in the NO + CO reaction on supported metal catalysts. The characteristic features of the formation of NC0 species in the high-temperature NO + CO reaction on supported Rh have previously been investigated and established only by infrared spectroscopy [6-131. As the nature of the supports exerted a dominant influence on the location of the NC0 absorption band and also on the reactivity of the NC0 [9,10], it was concluded that, in contrast to the earlier belief [6-81, the NC0 species yielding the intense absorption bands at 2210-2310 cm-’ is situated not on the Rh, but exclusively on the support. This conception has recently been accepted by Keller and Bell [ll]. In a study of HNCO adsorption on Rh surfaces, we found that two states exist on the clean surface: a physisorbed state and a strongly chemisorbed state. HNCO adsorbs molecularly on Rh(ll1) at 95 K, but dissociates at higher temperatures. The NC0 formed in this process is an unstable surface 0378-5963/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
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E Solymosi el al. / NC0
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species on the Rh. It starts to decompose at around 150 K, producing adsorbed N and CO, which desorb only at higher temperatures. Electron energy loss spectroscopic (EELS) measurements in the electronic range indicate that NC0 decomposes completely on Rh(l,ll) at 300-360 K [S]. The primary objective of the present study is to examine the effects of preadsorbed oxygen on the adsorption and stability of HNCO on the Rh(ll1) surface.
2. Experimental HNCO was prepared by the reaction of saturated aqueous HNCO solution with 95’7%H,PO, at 300 K. This route was proposed first by Ashby and Werner [14], who gave no experimental details. Since some experience is needed in the preparation of HNCO, we consider it useful to give a more detailed description of the preparation method. This might promote the study of the surface behavior of this interesting compound. The side-reactions which decrease HNCO production are hydrolysis, decomposition and polymerization. HNCO was produced in a vacuum apparatus consisting of a reaction vessel, 3 traps with taps and a pumping system. 5.68 g P,O, is dissolved in 2.5 ml doubly distilled water in the reaction vessel. 4.86 g KOCN is dissolved in 15 ml water and the solution is poured into a funnel connected by a joint to the vessel containing H,PO,. After evacuation of the system, the first trap is cooled with an acetone-dry ice mixture to 193 K. It is best to place the first trap as near as possible to the vessel. During continuous evacuation with a rotary pump, the KOCN solution is slowly introduced drop by drop into the H,PO, solution. The HNCO formed, together with some H,O is distilled over into the first trap. When the gas evolution becomes very vigorous, the introduction of KOCN should be stopped for a few moments. The best yield is obtained if this procedure lasts for 20-25 min. After completion of the reaction, the HNCO is vacuum-distilled at 228 K into the next trap, and cooled again to 193 K. It is advisable not to let the HNCO heat up above 228 K, because it then polymerizes quickly. HNCO is a colourless liquid at 193 K. About 1 ml liquid can be obtained by this method. The main contamination in the product is CO,. This can be eliminated by evacuation of the sample kept at 193 K for 3-5 min. After repetition of this distillation 2 or 3 times, the HNCO obtained should be stored in vacuum in a tube immersed in liquid air or nitrogen. At this temperature HNCO is a white solid. Further purification, if needed, can be performed under UHV conditions. For introduction of HNCO into the UHV chamber, the sample should be warmed to 193 K. It is useful, however, to evacuate its container at the temperature of liquid air and for a short period at 193 K too.
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The Rh(ll1) sample was the same as used previously [5]. As regards the details of the experimental methods and the cleaning procedure applied, we refer to our previous paper [5]. It is important to mention that after extensive cleaning the Auger spectrum of the Rh sample showed no signal due to boron contamination. Further, no boron segregation was observed during the heating of clean and HNCO-dosed Rh( 111) surfaces to - 1000 K.
3. Methods The experiments were carried out in a Varian ion pumped UHV system equipped with a single-pass CMA (PHI), a 3-grid retarding field analyzer (VG) and a quadrupole mass analyzer. The base pressure was 1.5 X 210-lo Torr. The heating rate in the thermal desorption measurements was - 10 K/s.
4. Results 4. I. EELS studies The adsorption of HNCO and the surface stability of NC0 formed was followed by EELS in the electronic range. It has recently been demonstrated in a number of cases that EEL spectroscopy can be successfully applied, at least as a fingerprint technique, to follow reactions on metal surfaces. From the intensities of the losses produced by adsorbates, no direct correlation exists as regards their surface concentration. However, the intensity changes are indicative of alterations in the surface concentration of the adsorbed species responsible for the losses. In the first experimental series the Rh surface was predosed with oxygen at 300 K, and HNCO was adsorbed on this surface at 110 K. The concentration of adsorbed oxygen was calculated from the relative 0 Auger signal R,,
Ro = hoS,2/RRh30*~ taking into account that at saturation the oxygen concentration is 8 x 10’4/cm2, which corresponds to 8, = 0.5 [15]. Some EEL spectra are shown in fig. 1. The features of the spectrum of clean Rh(ll1) agreed well with those reported in our previous studies [5,16]. We again found no losses at 7.9 and 8.7 eV, in contrast with the results mainly of optical measurements by other workers (see references in ref. [5]). As we noted earlier [5], in the majority of the previous measurements the vacuum conditions were poorer than in the present case, and the sample cleanliness was not checked by Auger spectroscopy. We are therefore inclined to think that in the previous studies the Rh samples were not sufficiently clean and the losses
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F. Solymosi et al. / NC0
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observed at 7.9 and at 8.7 eV were possibly caused by surface contaminants. Adsorption of 0, on the Rh(ll1) surface produced no new losses on the EEL spectrum of a clean Rh, but increased the intensities of the elastic peak and that of the losses at 5.2 and 6.7 eV. On exposure of this surface to HNCO at 110 K, two intense losses appeared, at 10.4 and 13.5 eV. In order to minimize the effect of the electron beam, we worked at the lowest possible beam current (I = 0.1 PA) and used a high spectrum-recording speed (2 eV/s). In addition, every spectrum taken was the result of separate HNCO exposure. The sample was cleaned by flushing at 1250 K before the new gas exposure. The intensities of both losses decreased only a little with the rise of oxygen concentration. This is illustrated by the intensity data in fig. 2a. A completely different picture was obtained when HNCO was adsorbed at 155 K, i.e. above the desorption temperature of weakly-held HNCO. On a clean surface, the intensities of the losses were less than at 110 K, but they increased with the rise of the surface oxygen concentration up to 8,, = 0.25, and then decreased (fig. 2~). These results suggest that, at least at certain oxygen concentrations, the number of NC0 species on Rh(ll1) is increased at 155 K. For this effect to be seen more clearly, the sample saturated with HNCO at the optimum con-
kecev,
-
20
I6
l2
8
L
0
II
Fig. 1. Electron energy loss spectra of Rh(ll1) surface coveted with adsorbates, (a) clean Rh(lll); (b) oxygen dosed surface, 0, = 0.25; (c) exposure of (b) td 30 L HNCO at 110 K; (d)-(f) after heating up (c) to different temperatures (E, = 70 eV, I = 0.1 PA).
F. Solymosi
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centration of adsorbed oxygen was heated up to different temperatures (the heating rate was - 10 K/s) and the intensity changes of the losses at 10.4 and 13.5 eV were determined at - 110 K. The sample was in all cases kept at a given temperature for 60 s: the intensity data then exhibited less scattering. Some spectra are drawn in fig. 1. For a reliable comparison, the measurements were repeated on a clean surface under exactly the same conditions. The data obtained are shown in fig. 3. It appears that the adsorbed oxygen basically influences the temperature-dependences of the intensities of the 10.4 and 13.5 eV losses. Although the initial intensity of the 10.4 eV loss on clean Rh(ll1) is slightly higher than on the oxygen-dosed surface, it markedly decreased during the heating, and disappeared completely at 330 K, a somewhat lower temperature than reported in our previous work [5]. The primary reason for this difference is that the sample was previously cooled immediately after attainment of the selected temperature, in contrast with the present case:The decrease in the intensity of the 10.4 eV loss, which is due exclusively to the surface isocyanate species, was much slower in the presence of adsorbed oxygen, and it vanished only above 380 K. As the chemisorbed CO formed in the surface dissociation of NC0 also gives a loss at 13,5 eV, the intensity changes of this loss are more complex (see discussion, section 5). In the study of the effect of the surface concentration of oxygen on the stability of NC0 losses at 300 K, we again found that both losses exhibited maximum intensity at 0, = 0.25. This is also shown in fig. 2b.
0
0
0 10.4 eV x 13.5 eV
Jf,J(lo-31
(4 6
0.1
0.3 a-
I
OS
Fig. 2. Effect of oxygen coverage on the intensities of 10.4 and 13.5 eV losses following HNCO ( - 36 L) adsorption at 110 K (a) and 155 K (c) and after heating the sample.exposed to HNCO at 110 to 300 K (b).
4
2
B
B
3. a .% \
F. Solymosi et al. / NC0
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4.2. Thermal desorption measurements Thermal desorption measurements showed that, following the adsorption of HNCO on the clean Rh(ll1) surface at 95-100 K, HNCO (T, = 130 and 200 K), H, (T, = 280 K), NH, ( TP = 415 K), CO ( TP = 450-480 K) and N, (T, = 670 and 790 K) desorbed [5]. Some characteristic thermal desorption spectra from oxygen-dosed surfaces are shown in fig. 4. No desorption of H, and NH, occurred, and the high-temperature peak attributed to the desorption of chemisorbed HNCO was also missing. This phenomenon was observed even at low oxygen concentrations, from 0, = 0.10. No change was experienced, however, in the low temperature peak for HNCO desorption. New products of the surface reaction were H,O (TP = 185 K) and CO,. The evolution of CO, occurred in two stages, TP = 360-320 K (p,) and 405 K (j3,). The amounts of both compounds increased with the rise of oxygen coverage. The optimum oxygen concentration for H,O was again at 0,~ 0.25. A gradual increase in the quantity of CO, was observed up to maximum oxygen concentration. At lower oxygen content, the /3, state developed first, but at higher oxygen concentration, when the amount of CO2 was significantly enhanced, more CO, formed in the /3, state. The peak temperature for p, was shifted to lower temperatures with increase of the oxygen coverage, while TP for & remained practically constant (fig. 5). The chemisorbed oxygen influenced the desorption of CO, the decomposition product of NCO. On a clean Rh(ll1) surface, after saturation with HNCO, the peak temperature, TP, for CO was 450 K. This value was shifted to lower temperatures with the increase of oxygen coverage up to 19, = 0.25, where
200
300
LOO
T(K)
I
‘3m
400
T(K)
500
Fig. 5. Effect of oxygen coverage on the desorption of CO2 and CO formed in the surface reaction following HNCO ( - 36 L) adsorption at 110 K. The curves are uncorrected for detection sensitivities.
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Tp = 397 K. With further rise of the oxygen coverage, however, Tp started to increase (fig. 5). As the data presented in fig. 6 show, the amount of CO desorbed hardly changed up to 0, = 0.25, but afterwards it decreased monotonically. Nitrogen desorbs from clean and oxygen-dosed surfaces in one broad peak. In the latter case the desorption begins again at lower temperatures (at the optimum oxygen concentration, 0, - 0.25, Tp = 606 K) than on the clean surface. In this case, however, the amount of desorbed N, is higher by a factor of 1.4-1.6 than on the clean surface. Characteristic TDS data are collected in table 1. It should be noted that the high-temperature desorption peak for N, ( Tp = 790 K) observed in our previous work [S] was missing. This confirms our assumption that this peak was caused by the segregation of a small amount of boron to the surface, and by the stabilization of the bonding of nitrogen to the surface. After the extensive cleaning and long use of this Rh(ll1) crystal, the level of boron in the Rh was drastically decreased and the segregation process became negligible even at higher temperatures. In harmony with this, no high-temperature peak for nitrogen was measured for this sample following nitrogen atom adsorption [17]. The surface coverage of NC0 on a clean Rh surface was calculated by comparing the amount of CO desorbed from the surface saturated with HNCO
Fig. 6. Areas of CO and CO, desorption exposed to 36 L HNCO at 110 K.
peaks
as a function
of oxygen
coverage
on Rh(ll1)
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Table 1 Summary of the results of TDS measurements; the oxygen coverage was 0, = 0.25 E ‘) (kJ/mol)
Refs.
State
TPW)
T, (K)
HNCO/HNCO
130
110
34
H,O/H,O H,O/HNCO
191 185
100 110
46
CO, /CO, CO,(&)/HNCO CO/HNCO co/co N,/HNCO
225-290 349 405 397 480 606
110 110 110 110 300,110 110
56 87 101 99 120 [19,20] 143
I201 Present Present Present
N,/N
603
300
142
[I71
CO,( P2)/HNCO
Present
PI Present
Present
‘) Calculated from the observed values of Tp with a preexponential factor of lOI s-’
with the value obtained after saturation with CO alone. The surface concentration of CO on Rh(lll) at saturation was 1.2 X 1015 molecules CO/cm* [19]. In this way we obtained 5 x lOI molecules CO/cm*. Taking into account eq. (6) (see next section), this means that the same amount of NC0 bonded irreversibly to the Rh. On oxygen-dosed surfaces the amount of CO was practically the same up to 0, = 025. As a result of surface oxidation, however, CO, is also formed. Quantitative analysis of the peak areas, with allowance for the pumping rate and mass spectrometer sensitivity for CO*, indicated a value of 1.7 x 1Or4 molecules CO,/cm* at 13, = 0.25. (This latter value, however, is inherently uncertain, f 50%, as the effective pumping speed and gauge calibration factor is not precisely defined.) Accordingly, the maximum attainable surface concentration of irreversibly adsorbed NC0 following HNCO adsorption on oxygen-dosed Rh(lll) at 110 K is about 6.7 X 1014 NC0 molecules/cd. This value is somewhat lower than expected from the increased amount of N, desorbed from an oxygen-dosed surface. A possible reason for this difference is that on the clean surface some N(,, is consumed in the formation of NH, [5].
5. Discussion 5. I. Adsorption,
desorption and reaction of HNCO
HNCO adsorbs readily onto the clean Rh(ll1) surface at 100 K, with a high sticking probability, and produces two adsorption states, CI(physisorption) and j3 (chemisorption) [5]. In contrast with the adsorption of CH,OH on the same Rh(ll1) surface [18], the development of the two stages occurred almost simultaneously and attempts to detect another low-temperature stage indica-
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on Rh(l I I) surfaces
tive of the formation of the condensed layer of HNCO were unsuccessful. As regards the origin of the /3 stage, two assumptions can be proposed: (i) a strongly-bonded HNCO is formed on the surface, and is desorbed intact above 200 K; (ii) HNCO adsorbs dissociatively on the clean Rh surface, HNCO,,,
= H(,, + NCO,,, ,
or the adsorbed
HNCO
(1)
dissociates
into adsorbed
H and NC0
at above 100 K,
,
HNCO(,,
= HNCO,,,
HNCO,,,
= H(,, + NCO,,, ,
(2)
and in the p stage the NC0 NCO(,, + H,,, = HNCO,,,
(3) species recombines
,
with adsorbed
H atoms,(41
and HNCO desorbs. The presence of adsorbed oxygen exerted no apparent influence on the amount of HNCO desorbed in the (Y stage, but it greatly decreased the development of the j3 stage. This effect of preadsorbed oxygen can be more easily interpreted in terms of the occurrence of the surface dissociation of HNCO (case ii). We may consider that, the adsorbed oxygen reacts with the hydrogen of HNCO, yielding adsorbed OH and NCO, Oca, + H-HNCO,,,
= OH,,, + NCO,,, .
(5)
The formation of a strong O-H bond reduces the extent of the associative reaction of H and NC0 (step 4) and, as demonstrated, leads at a certain oxygen concentration to the complete cessation of this reaction. Instead of this process, the OH groups recombine and H,O is produced at TP = 185 K, i.e. far below the desorption temperature of the fl state for HNCO. Another consequence of the consumption of adsorbed H in this process is the absence of the formation of NH, in reactions of adsorbed HNCO and N with H. The elimination of the associative desorption of HNCO and the formation of NH, result in a higher surface concentration of NCO. This is exhibited in the increased amounts of N, and CO + CO, formed in the surface dissociation of NCO, (6)
NCO,,, = N@, + CO,,, 7 or in its reaction
with chemisorbed
NCO,,, + Oca, + NC,, + COX,, 9 at higher From N, from that the
oxygen, (7)
temperatures. a comparison of the characteristics of the desorption of CO, CO, and the clean and oxygen-dosed Rh surfaces (table l), it can be concluded formation of CO and N, is not reaction rate-limited, but desorption
F. Solymosi et al. / NC0 on Rh(l I I) surfaces
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rate-limited. Nevertheless, the desorption of both CO and N, proceed at a somewhat lower temperature than from the clean surface following either the adsorption of CO [19] and atomic N [17] or the adsorption of HNCO [5] on Rh(ll1) surfaces. In the case of CO, where no measurable change occurred in the amount of CO desorbed up to 19, = 0.25, it appears that in the presence of adsorbed 0 and N produced as in eq. 6, CO tends to form a weaker bond with the Rh; this develops only at high CO exposures on a clean Rh(ll1) surface [19]. The weakening effect of adsorbed oxygens on the metal-O bond has been observed in many systems [27]. As the adsorbed CO, desorbs from Rh(ll1) surface at lower temperatures than in the present case (TP decreases from 290 to 225 K with the increase of the CO, coverage [20]) its formation is very probably a reaction rate-limited process. The primary process responsible for the CO, evolution is the reaction of chemisorbed CO with 0, co,,,
= O@, = CO,&, .
(8)
The peak temperature for the & state of CO, evolution agrees quite well with that found in the study of reaction (8) on Rh wire [21]. As regards the p, state of CO, formation the situation is more complex. It is very likely that the dominant process is the same as in the case of the & state as calculation showed that reaction (8) may occur below 300 K, too [22]. However, we can not automatically rule out the contribution of the direct oxidation of adsorbed NC0 (eq. (7)). The occurrence of this reaction was observed on Cu(ll1) surface above 400 K where the presence of adsorbed CO was excluded [2]. The results of EELS measurements support the above conclusions and give a somewhat more detailed picture. As established before, adsorbed isocyanate groups gives two intense losses at 10.4 and 13.5 ev in the EEL spectrum in the electronic range [2,4,5]. With this technique, however, it is not possible to distinguish between undissociated and dissociated HNCO. This is in contrast with transmission infrared spectroscopy and vibrational EELS, where the asymmetric stretching frequency of HNCO appears at 2260 cm-’ and that of NC0 on Pt and Rh at 2160-2180 cm-’ [3,12,23,24]. EEL spectra taken following HNCO adsorption at 110 K on oxygen-dosed surfaces disclosed on significant effect of adsorbed oxygen. However, when the adsorption was performed at 155 K, i.e. above the desorption temperature of a-HNCO, but below the decomposition temperature of adsorbed NCO, the intensities of both losses were greater than on a clean surface (with the exception of samples with high oxygen coverage), and showed a maximum at (?o = 0.25. these results suggest that preahorbed oxygen promotes the chemisorption of HNCO on Rh(lll), and very probably the dissociative adsorption of HNCO by the formation of OH groups. Accordingly the role of adsorbed oxygen is not merely the elimination of the associative desorption of HNCO (eq. (4)), as discussed before. This effect of adsorbed oxygen was clearly demon-
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on Rh(lll)
w/aces
strated on Cu(lll), where no HNCO uptake was observed at all at 300 K in the absence of adsorbed oxygen [1,2]. The decrease in the intensities of NC0 losses at higher oxygen concentration (above 0, = 0.25) can obviously be attributed to the diminution of surface sites available for the bonding of NC0 species. 5.2. Stability
of NC0
on oxygen-dosed
Rh(lll)
Let us now discuss the effect of adsorbed oxygen on the stability of the NC0 species. In this respect, the intensity changes of the losses observed during the successive heating of HNCO-covered surfaces are of importance (fig. 3). In the interpretation of these results we have to take into account that CO adsorbed on Rh also gives a loss at 13.5 eV [5,16,20]. For convenience, in fig. 3 we have denoted three different stages. In harmony with the thermal desorption data, the rapid decrease in the intensities of the 10.4 and 13.5 ev losses in stage 4 can be attributed on both surfaces to the desorption of weakly-held HNCO. In stage B, on the clean surface, when the intensities of the 10.4 and 13.5 eV losses change in opposite direction, the desorption of more strongly-held HNCO and the decomposition of NC0 (step 4) occur. On the oxygen-dosed surface no desorption of HNCO contributes to the recorded changes in the EEL spectra. The reason that the intensity of the 13.5 eV loss increases in this temperature range is that the chemisorbed CO formed produces a more intense loss than that of chemisorbed NCO, and thus its formation overcompensates the lessening effect of the decomposition of NC0 on the intensity of the 13.5 eV loss. Stage C on a clean surface is characterized by the desorption of adsorbed CO formed in NC0 decomposition. On the oxygen-dosed surface, reaction between adsorbed CO and 0 also takes place. It is most important to mention that the decomposition of NC0 species proceeds more slowly in stage B on oxygen-dosed Rh, as indicated by the smaller decay in intensity of the 10.4 eV loss, and by a shift in the onset-temperature of the intensity increase of the 13.5 eV loss to a higher value, where it exhibited a sharp maximum at around 335 K. The NC0 loss at 10.4 eV in the presence of adsorbed oxygen can easily be detected even at 370 K, demonstrating that preadsorbed oxygen markedly increases the stability of the NC0 complex bonded to the Rh. Recently, a similar result was obtained on Pt(lOO) by vibrational EELS measurements [24]. In this case it was possible to establish that preadsorbed oxygen caused a shift in v,,(NCO) towards higher frequency. This may indicate that the primary reason for the stabilizing effect of adsorbed oxygen is that, through a ligand effect, it results in a more ionic bond and thereby in the higher stability of Rh-NCO. As this observation may shed new light on the role of the NC0 complex in
F. Solymosi et al. / NC0 on Rhjl II) surfaces
245
the NO + CO reaction, it was important to determine whether this stabilizing effect of adsorbed oxygen holds under conditions more closely approaching those prevailing during the catalytic reduction of NO, i.e. elevated pressure and supported catalyst. The NC0 species was produced in this case too by the adsorption of HNCO at 200-300 K. The data obtained so far on silica-supported Pt and Rh [26] showed good agreement with those measured on single-crystal surfaces under UHV conditions.
References [I] [2] [3] [4] [5] [6] [7] [S] [9] [lo] (111 [12] [13] [14] [15] (161 [17] (181 [19] [20] [21] [22] [23] (241 [25] [26] [27]
F. Solymosi and J. Kiss, in: Proc. IVC-8, ICSS-4, ECOSS-3, Cannes, 1980. p. 213. F. Solymosi and J. Kiss, Surface Sci. 104 (1981) 181. R.j. Gorte, L.D. Schmidt and B.A. Sexton, J. Catalysis 67 (1981) 387. F. Solymosi and J. Kiss, Surface Sci. 108 (1981) 368. J. Kiss and F. Solymosi, Surface Sci. 135 (1983) 243. M.L. Unland, J. Catalysis 31 (1973) 459. H. Arai and H. Tominaga. J. Catalysis 43 (1976) 131. R. Nakamura, R. Nakai. K. Sugiyama and E. Echigoya, Bull. Chem. Sot. Japan 54 (1981) 1950. F. Solymosi and J. SPrkPny, Appl. Surface Sci. 3 (1976) 68. F. Solymosi, L. Volgyesi and J. Raskb, 2. Physik. Chem. (NF) 120 (1980) 79. W.C. Keller and A.T. Bell. J. Catalysis 84 (1983) 200. F. Solymosi and J. Rasko, unpublished results. J. Rasko, L. Volgyesi, M. Lancz and F. Solymosi, in: Proc. 8th Intern. Congr. on Catalysis, Berlin, 1984, in press. R.A. Ashby and R.L. Werner, J. Mol. Spectrosc. 18 (1965) 184. P.A. Thiel, J.T. Yates, Jr. and W.H. Weinberg, Surface Sci. 82 (1979) 45. F. Solymosi and J. Kiss, J. Catalysis 81 (1983) 95. A. Berko, Thesis, University of Szeged (1983). F. Solymosi, A. Berko and T.I. Tarn&xi, Surface Sci. 141 (1984) 533. D.G. Castner, B.A. Sexton and G.A. Somorjai, Surface Sci. 71 (1978) 519; P.A. Thiel, A.D. Williams, J.T. Yates and W.H. Weinberg, Surface Sci. 84 (1979) 54. J. Kiss and F. Solymosi, unpublished results. CT. Campbell, S.K. Shi and J.M. White, Appl. Surface Sci. 2 (1979) 382. L.H. Dubois and G.A. Somorjai, Surface Sci. 128 (1983) L231. J. Rasko and F. Solymosi, J. Catalysis 75 (1982) 78. M. Surman. F. Solymosi, P. Hoffmann and D.A. King, J. Catalysis, in press. J.J. Zinck and W.H. Weinberg, J. Vacuum Sci. Technol. 17 (1980) 188. F. Solymosi and J. Rasko. J. Appl. Catalysis 10 (1984) 19. G. Ertl, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis. Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) p. 82.
233
EFFECTS OF PREADSORBED OXYGEN ON THE FORMATION DECOMPOSITION OF NC0 ON Rh’(111) SURFACES F. SOLYMOSI,
A. BERKb
AND
and T.I. TARNCXZI
Reaction Kinetics Research Group, The University, P.O. Box 105, H -6701 Szeged, Hungary Received 26 January 1984; accepted for publication 8 May 1984
The interaction of HNCO with oxygen dosed Rh(ll1) surface has been investigated by Auger electron, electron energy loss and thermal desorption spectroscopy. The presence of adsorbed oxygen exerted no apparent influence on the weakly adsorbed HNCO (Tp = 130 K). It promoted, however, the dissociative adsorption of HNCO by forming a strong O-H bond which prevented the associative desorption of HNCO. As a result no H, and NH, formation occurred, in contrast with the clean surface, and the surface concentration of irreversibly bonded NC0 was also increased. New products of the surface reaction were Ha0 and CO,. in addition to CO and N, observed on a clean surface. From the behavior of the losses characteristic for the adsorbed NC0 it appeared that the preadsorbed oxygen exerted a significant stabilizing effect on the NC0 bonded to the Rh.
1.
Introduction
The behavior of HNCO on clean metals has been investigated so far on Cu(ll1) [1,2], Pt(ll1) [3], Pt(ll0) [4] and Rh(ll1) [5] surfaces. The primary motivation of this work was to provide data for a better understanding of the surface chemistry of NC0 intermediates formed in the NO + CO reaction on supported metal catalysts. The characteristic features of the formation of NC0 species in the high-temperature NO + CO reaction on supported Rh have previously been investigated and established only by infrared spectroscopy [6-131. As the nature of the supports exerted a dominant influence on the location of the NC0 absorption band and also on the reactivity of the NC0 [9,10], it was concluded that, in contrast to the earlier belief [6-81, the NC0 species yielding the intense absorption bands at 2210-2310 cm-’ is situated not on the Rh, but exclusively on the support. This conception has recently been accepted by Keller and Bell [ll]. In a study of HNCO adsorption on Rh surfaces, we found that two states exist on the clean surface: a physisorbed state and a strongly chemisorbed state. HNCO adsorbs molecularly on Rh(ll1) at 95 K, but dissociates at higher temperatures. The NC0 formed in this process is an unstable surface 0378-5963/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
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E Solymosi el al. / NC0
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surf&es
species on the Rh. It starts to decompose at around 150 K, producing adsorbed N and CO, which desorb only at higher temperatures. Electron energy loss spectroscopic (EELS) measurements in the electronic range indicate that NC0 decomposes completely on Rh(l,ll) at 300-360 K [S]. The primary objective of the present study is to examine the effects of preadsorbed oxygen on the adsorption and stability of HNCO on the Rh(ll1) surface.
2. Experimental HNCO was prepared by the reaction of saturated aqueous HNCO solution with 95’7%H,PO, at 300 K. This route was proposed first by Ashby and Werner [14], who gave no experimental details. Since some experience is needed in the preparation of HNCO, we consider it useful to give a more detailed description of the preparation method. This might promote the study of the surface behavior of this interesting compound. The side-reactions which decrease HNCO production are hydrolysis, decomposition and polymerization. HNCO was produced in a vacuum apparatus consisting of a reaction vessel, 3 traps with taps and a pumping system. 5.68 g P,O, is dissolved in 2.5 ml doubly distilled water in the reaction vessel. 4.86 g KOCN is dissolved in 15 ml water and the solution is poured into a funnel connected by a joint to the vessel containing H,PO,. After evacuation of the system, the first trap is cooled with an acetone-dry ice mixture to 193 K. It is best to place the first trap as near as possible to the vessel. During continuous evacuation with a rotary pump, the KOCN solution is slowly introduced drop by drop into the H,PO, solution. The HNCO formed, together with some H,O is distilled over into the first trap. When the gas evolution becomes very vigorous, the introduction of KOCN should be stopped for a few moments. The best yield is obtained if this procedure lasts for 20-25 min. After completion of the reaction, the HNCO is vacuum-distilled at 228 K into the next trap, and cooled again to 193 K. It is advisable not to let the HNCO heat up above 228 K, because it then polymerizes quickly. HNCO is a colourless liquid at 193 K. About 1 ml liquid can be obtained by this method. The main contamination in the product is CO,. This can be eliminated by evacuation of the sample kept at 193 K for 3-5 min. After repetition of this distillation 2 or 3 times, the HNCO obtained should be stored in vacuum in a tube immersed in liquid air or nitrogen. At this temperature HNCO is a white solid. Further purification, if needed, can be performed under UHV conditions. For introduction of HNCO into the UHV chamber, the sample should be warmed to 193 K. It is useful, however, to evacuate its container at the temperature of liquid air and for a short period at 193 K too.
E Solymosi er al. / NC0
on Rh(ll
I) surfaces
235
The Rh(ll1) sample was the same as used previously [5]. As regards the details of the experimental methods and the cleaning procedure applied, we refer to our previous paper [5]. It is important to mention that after extensive cleaning the Auger spectrum of the Rh sample showed no signal due to boron contamination. Further, no boron segregation was observed during the heating of clean and HNCO-dosed Rh( 111) surfaces to - 1000 K.
3. Methods The experiments were carried out in a Varian ion pumped UHV system equipped with a single-pass CMA (PHI), a 3-grid retarding field analyzer (VG) and a quadrupole mass analyzer. The base pressure was 1.5 X 210-lo Torr. The heating rate in the thermal desorption measurements was - 10 K/s.
4. Results 4. I. EELS studies The adsorption of HNCO and the surface stability of NC0 formed was followed by EELS in the electronic range. It has recently been demonstrated in a number of cases that EEL spectroscopy can be successfully applied, at least as a fingerprint technique, to follow reactions on metal surfaces. From the intensities of the losses produced by adsorbates, no direct correlation exists as regards their surface concentration. However, the intensity changes are indicative of alterations in the surface concentration of the adsorbed species responsible for the losses. In the first experimental series the Rh surface was predosed with oxygen at 300 K, and HNCO was adsorbed on this surface at 110 K. The concentration of adsorbed oxygen was calculated from the relative 0 Auger signal R,,
Ro = hoS,2/RRh30*~ taking into account that at saturation the oxygen concentration is 8 x 10’4/cm2, which corresponds to 8, = 0.5 [15]. Some EEL spectra are shown in fig. 1. The features of the spectrum of clean Rh(ll1) agreed well with those reported in our previous studies [5,16]. We again found no losses at 7.9 and 8.7 eV, in contrast with the results mainly of optical measurements by other workers (see references in ref. [5]). As we noted earlier [5], in the majority of the previous measurements the vacuum conditions were poorer than in the present case, and the sample cleanliness was not checked by Auger spectroscopy. We are therefore inclined to think that in the previous studies the Rh samples were not sufficiently clean and the losses
236
F. Solymosi et al. / NC0
on Rh(ll1)
surJaces
observed at 7.9 and at 8.7 eV were possibly caused by surface contaminants. Adsorption of 0, on the Rh(ll1) surface produced no new losses on the EEL spectrum of a clean Rh, but increased the intensities of the elastic peak and that of the losses at 5.2 and 6.7 eV. On exposure of this surface to HNCO at 110 K, two intense losses appeared, at 10.4 and 13.5 eV. In order to minimize the effect of the electron beam, we worked at the lowest possible beam current (I = 0.1 PA) and used a high spectrum-recording speed (2 eV/s). In addition, every spectrum taken was the result of separate HNCO exposure. The sample was cleaned by flushing at 1250 K before the new gas exposure. The intensities of both losses decreased only a little with the rise of oxygen concentration. This is illustrated by the intensity data in fig. 2a. A completely different picture was obtained when HNCO was adsorbed at 155 K, i.e. above the desorption temperature of weakly-held HNCO. On a clean surface, the intensities of the losses were less than at 110 K, but they increased with the rise of the surface oxygen concentration up to 8,, = 0.25, and then decreased (fig. 2~). These results suggest that, at least at certain oxygen concentrations, the number of NC0 species on Rh(ll1) is increased at 155 K. For this effect to be seen more clearly, the sample saturated with HNCO at the optimum con-
kecev,
-
20
I6
l2
8
L
0
II
Fig. 1. Electron energy loss spectra of Rh(ll1) surface coveted with adsorbates, (a) clean Rh(lll); (b) oxygen dosed surface, 0, = 0.25; (c) exposure of (b) td 30 L HNCO at 110 K; (d)-(f) after heating up (c) to different temperatures (E, = 70 eV, I = 0.1 PA).
F. Solymosi
et al. / NC0
on Rh(lll)
surfaces
231
centration of adsorbed oxygen was heated up to different temperatures (the heating rate was - 10 K/s) and the intensity changes of the losses at 10.4 and 13.5 eV were determined at - 110 K. The sample was in all cases kept at a given temperature for 60 s: the intensity data then exhibited less scattering. Some spectra are drawn in fig. 1. For a reliable comparison, the measurements were repeated on a clean surface under exactly the same conditions. The data obtained are shown in fig. 3. It appears that the adsorbed oxygen basically influences the temperature-dependences of the intensities of the 10.4 and 13.5 eV losses. Although the initial intensity of the 10.4 eV loss on clean Rh(ll1) is slightly higher than on the oxygen-dosed surface, it markedly decreased during the heating, and disappeared completely at 330 K, a somewhat lower temperature than reported in our previous work [5]. The primary reason for this difference is that the sample was previously cooled immediately after attainment of the selected temperature, in contrast with the present case:The decrease in the intensity of the 10.4 eV loss, which is due exclusively to the surface isocyanate species, was much slower in the presence of adsorbed oxygen, and it vanished only above 380 K. As the chemisorbed CO formed in the surface dissociation of NC0 also gives a loss at 13,5 eV, the intensity changes of this loss are more complex (see discussion, section 5). In the study of the effect of the surface concentration of oxygen on the stability of NC0 losses at 300 K, we again found that both losses exhibited maximum intensity at 0, = 0.25. This is also shown in fig. 2b.
0
0
0 10.4 eV x 13.5 eV
Jf,J(lo-31
(4 6
0.1
0.3 a-
I
OS
Fig. 2. Effect of oxygen coverage on the intensities of 10.4 and 13.5 eV losses following HNCO ( - 36 L) adsorption at 110 K (a) and 155 K (c) and after heating the sample.exposed to HNCO at 110 to 300 K (b).
4
2
B
B
3. a .% \
F. Solymosi et al. / NC0
on Rh(l I I) surfaces
239
4.2. Thermal desorption measurements Thermal desorption measurements showed that, following the adsorption of HNCO on the clean Rh(ll1) surface at 95-100 K, HNCO (T, = 130 and 200 K), H, (T, = 280 K), NH, ( TP = 415 K), CO ( TP = 450-480 K) and N, (T, = 670 and 790 K) desorbed [5]. Some characteristic thermal desorption spectra from oxygen-dosed surfaces are shown in fig. 4. No desorption of H, and NH, occurred, and the high-temperature peak attributed to the desorption of chemisorbed HNCO was also missing. This phenomenon was observed even at low oxygen concentrations, from 0, = 0.10. No change was experienced, however, in the low temperature peak for HNCO desorption. New products of the surface reaction were H,O (TP = 185 K) and CO,. The evolution of CO, occurred in two stages, TP = 360-320 K (p,) and 405 K (j3,). The amounts of both compounds increased with the rise of oxygen coverage. The optimum oxygen concentration for H,O was again at 0,~ 0.25. A gradual increase in the quantity of CO, was observed up to maximum oxygen concentration. At lower oxygen content, the /3, state developed first, but at higher oxygen concentration, when the amount of CO2 was significantly enhanced, more CO, formed in the /3, state. The peak temperature for p, was shifted to lower temperatures with increase of the oxygen coverage, while TP for & remained practically constant (fig. 5). The chemisorbed oxygen influenced the desorption of CO, the decomposition product of NCO. On a clean Rh(ll1) surface, after saturation with HNCO, the peak temperature, TP, for CO was 450 K. This value was shifted to lower temperatures with the increase of oxygen coverage up to 19, = 0.25, where
200
300
LOO
T(K)
I
‘3m
400
T(K)
500
Fig. 5. Effect of oxygen coverage on the desorption of CO2 and CO formed in the surface reaction following HNCO ( - 36 L) adsorption at 110 K. The curves are uncorrected for detection sensitivities.
240
F. Solymosi et al. / NC0
on Rh(1 I I) surfaces
Tp = 397 K. With further rise of the oxygen coverage, however, Tp started to increase (fig. 5). As the data presented in fig. 6 show, the amount of CO desorbed hardly changed up to 0, = 0.25, but afterwards it decreased monotonically. Nitrogen desorbs from clean and oxygen-dosed surfaces in one broad peak. In the latter case the desorption begins again at lower temperatures (at the optimum oxygen concentration, 0, - 0.25, Tp = 606 K) than on the clean surface. In this case, however, the amount of desorbed N, is higher by a factor of 1.4-1.6 than on the clean surface. Characteristic TDS data are collected in table 1. It should be noted that the high-temperature desorption peak for N, ( Tp = 790 K) observed in our previous work [S] was missing. This confirms our assumption that this peak was caused by the segregation of a small amount of boron to the surface, and by the stabilization of the bonding of nitrogen to the surface. After the extensive cleaning and long use of this Rh(ll1) crystal, the level of boron in the Rh was drastically decreased and the segregation process became negligible even at higher temperatures. In harmony with this, no high-temperature peak for nitrogen was measured for this sample following nitrogen atom adsorption [17]. The surface coverage of NC0 on a clean Rh surface was calculated by comparing the amount of CO desorbed from the surface saturated with HNCO
Fig. 6. Areas of CO and CO, desorption exposed to 36 L HNCO at 110 K.
peaks
as a function
of oxygen
coverage
on Rh(ll1)
F. Solymosi et al. / NC0
on Rh(Ill)
surfaces
241
Table 1 Summary of the results of TDS measurements; the oxygen coverage was 0, = 0.25 E ‘) (kJ/mol)
Refs.
State
TPW)
T, (K)
HNCO/HNCO
130
110
34
H,O/H,O H,O/HNCO
191 185
100 110
46
CO, /CO, CO,(&)/HNCO CO/HNCO co/co N,/HNCO
225-290 349 405 397 480 606
110 110 110 110 300,110 110
56 87 101 99 120 [19,20] 143
I201 Present Present Present
N,/N
603
300
142
[I71
CO,( P2)/HNCO
Present
PI Present
Present
‘) Calculated from the observed values of Tp with a preexponential factor of lOI s-’
with the value obtained after saturation with CO alone. The surface concentration of CO on Rh(lll) at saturation was 1.2 X 1015 molecules CO/cm* [19]. In this way we obtained 5 x lOI molecules CO/cm*. Taking into account eq. (6) (see next section), this means that the same amount of NC0 bonded irreversibly to the Rh. On oxygen-dosed surfaces the amount of CO was practically the same up to 0, = 025. As a result of surface oxidation, however, CO, is also formed. Quantitative analysis of the peak areas, with allowance for the pumping rate and mass spectrometer sensitivity for CO*, indicated a value of 1.7 x 1Or4 molecules CO,/cm* at 13, = 0.25. (This latter value, however, is inherently uncertain, f 50%, as the effective pumping speed and gauge calibration factor is not precisely defined.) Accordingly, the maximum attainable surface concentration of irreversibly adsorbed NC0 following HNCO adsorption on oxygen-dosed Rh(lll) at 110 K is about 6.7 X 1014 NC0 molecules/cd. This value is somewhat lower than expected from the increased amount of N, desorbed from an oxygen-dosed surface. A possible reason for this difference is that on the clean surface some N(,, is consumed in the formation of NH, [5].
5. Discussion 5. I. Adsorption,
desorption and reaction of HNCO
HNCO adsorbs readily onto the clean Rh(ll1) surface at 100 K, with a high sticking probability, and produces two adsorption states, CI(physisorption) and j3 (chemisorption) [5]. In contrast with the adsorption of CH,OH on the same Rh(ll1) surface [18], the development of the two stages occurred almost simultaneously and attempts to detect another low-temperature stage indica-
F. Solymosi et al. / NC0
242
on Rh(l I I) surfaces
tive of the formation of the condensed layer of HNCO were unsuccessful. As regards the origin of the /3 stage, two assumptions can be proposed: (i) a strongly-bonded HNCO is formed on the surface, and is desorbed intact above 200 K; (ii) HNCO adsorbs dissociatively on the clean Rh surface, HNCO,,,
= H(,, + NCO,,, ,
or the adsorbed
HNCO
(1)
dissociates
into adsorbed
H and NC0
at above 100 K,
,
HNCO(,,
= HNCO,,,
HNCO,,,
= H(,, + NCO,,, ,
(2)
and in the p stage the NC0 NCO(,, + H,,, = HNCO,,,
(3) species recombines
,
with adsorbed
H atoms,(41
and HNCO desorbs. The presence of adsorbed oxygen exerted no apparent influence on the amount of HNCO desorbed in the (Y stage, but it greatly decreased the development of the j3 stage. This effect of preadsorbed oxygen can be more easily interpreted in terms of the occurrence of the surface dissociation of HNCO (case ii). We may consider that, the adsorbed oxygen reacts with the hydrogen of HNCO, yielding adsorbed OH and NCO, Oca, + H-HNCO,,,
= OH,,, + NCO,,, .
(5)
The formation of a strong O-H bond reduces the extent of the associative reaction of H and NC0 (step 4) and, as demonstrated, leads at a certain oxygen concentration to the complete cessation of this reaction. Instead of this process, the OH groups recombine and H,O is produced at TP = 185 K, i.e. far below the desorption temperature of the fl state for HNCO. Another consequence of the consumption of adsorbed H in this process is the absence of the formation of NH, in reactions of adsorbed HNCO and N with H. The elimination of the associative desorption of HNCO and the formation of NH, result in a higher surface concentration of NCO. This is exhibited in the increased amounts of N, and CO + CO, formed in the surface dissociation of NCO, (6)
NCO,,, = N@, + CO,,, 7 or in its reaction
with chemisorbed
NCO,,, + Oca, + NC,, + COX,, 9 at higher From N, from that the
oxygen, (7)
temperatures. a comparison of the characteristics of the desorption of CO, CO, and the clean and oxygen-dosed Rh surfaces (table l), it can be concluded formation of CO and N, is not reaction rate-limited, but desorption
F. Solymosi et al. / NC0 on Rh(l I I) surfaces
243
rate-limited. Nevertheless, the desorption of both CO and N, proceed at a somewhat lower temperature than from the clean surface following either the adsorption of CO [19] and atomic N [17] or the adsorption of HNCO [5] on Rh(ll1) surfaces. In the case of CO, where no measurable change occurred in the amount of CO desorbed up to 19, = 0.25, it appears that in the presence of adsorbed 0 and N produced as in eq. 6, CO tends to form a weaker bond with the Rh; this develops only at high CO exposures on a clean Rh(ll1) surface [19]. The weakening effect of adsorbed oxygens on the metal-O bond has been observed in many systems [27]. As the adsorbed CO, desorbs from Rh(ll1) surface at lower temperatures than in the present case (TP decreases from 290 to 225 K with the increase of the CO, coverage [20]) its formation is very probably a reaction rate-limited process. The primary process responsible for the CO, evolution is the reaction of chemisorbed CO with 0, co,,,
= O@, = CO,&, .
(8)
The peak temperature for the & state of CO, evolution agrees quite well with that found in the study of reaction (8) on Rh wire [21]. As regards the p, state of CO, formation the situation is more complex. It is very likely that the dominant process is the same as in the case of the & state as calculation showed that reaction (8) may occur below 300 K, too [22]. However, we can not automatically rule out the contribution of the direct oxidation of adsorbed NC0 (eq. (7)). The occurrence of this reaction was observed on Cu(ll1) surface above 400 K where the presence of adsorbed CO was excluded [2]. The results of EELS measurements support the above conclusions and give a somewhat more detailed picture. As established before, adsorbed isocyanate groups gives two intense losses at 10.4 and 13.5 ev in the EEL spectrum in the electronic range [2,4,5]. With this technique, however, it is not possible to distinguish between undissociated and dissociated HNCO. This is in contrast with transmission infrared spectroscopy and vibrational EELS, where the asymmetric stretching frequency of HNCO appears at 2260 cm-’ and that of NC0 on Pt and Rh at 2160-2180 cm-’ [3,12,23,24]. EEL spectra taken following HNCO adsorption at 110 K on oxygen-dosed surfaces disclosed on significant effect of adsorbed oxygen. However, when the adsorption was performed at 155 K, i.e. above the desorption temperature of a-HNCO, but below the decomposition temperature of adsorbed NCO, the intensities of both losses were greater than on a clean surface (with the exception of samples with high oxygen coverage), and showed a maximum at (?o = 0.25. these results suggest that preahorbed oxygen promotes the chemisorption of HNCO on Rh(lll), and very probably the dissociative adsorption of HNCO by the formation of OH groups. Accordingly the role of adsorbed oxygen is not merely the elimination of the associative desorption of HNCO (eq. (4)), as discussed before. This effect of adsorbed oxygen was clearly demon-
F. Solymosi ef al. / NC0
244
on Rh(lll)
w/aces
strated on Cu(lll), where no HNCO uptake was observed at all at 300 K in the absence of adsorbed oxygen [1,2]. The decrease in the intensities of NC0 losses at higher oxygen concentration (above 0, = 0.25) can obviously be attributed to the diminution of surface sites available for the bonding of NC0 species. 5.2. Stability
of NC0
on oxygen-dosed
Rh(lll)
Let us now discuss the effect of adsorbed oxygen on the stability of the NC0 species. In this respect, the intensity changes of the losses observed during the successive heating of HNCO-covered surfaces are of importance (fig. 3). In the interpretation of these results we have to take into account that CO adsorbed on Rh also gives a loss at 13.5 eV [5,16,20]. For convenience, in fig. 3 we have denoted three different stages. In harmony with the thermal desorption data, the rapid decrease in the intensities of the 10.4 and 13.5 ev losses in stage 4 can be attributed on both surfaces to the desorption of weakly-held HNCO. In stage B, on the clean surface, when the intensities of the 10.4 and 13.5 eV losses change in opposite direction, the desorption of more strongly-held HNCO and the decomposition of NC0 (step 4) occur. On the oxygen-dosed surface no desorption of HNCO contributes to the recorded changes in the EEL spectra. The reason that the intensity of the 13.5 eV loss increases in this temperature range is that the chemisorbed CO formed produces a more intense loss than that of chemisorbed NCO, and thus its formation overcompensates the lessening effect of the decomposition of NC0 on the intensity of the 13.5 eV loss. Stage C on a clean surface is characterized by the desorption of adsorbed CO formed in NC0 decomposition. On the oxygen-dosed surface, reaction between adsorbed CO and 0 also takes place. It is most important to mention that the decomposition of NC0 species proceeds more slowly in stage B on oxygen-dosed Rh, as indicated by the smaller decay in intensity of the 10.4 eV loss, and by a shift in the onset-temperature of the intensity increase of the 13.5 eV loss to a higher value, where it exhibited a sharp maximum at around 335 K. The NC0 loss at 10.4 eV in the presence of adsorbed oxygen can easily be detected even at 370 K, demonstrating that preadsorbed oxygen markedly increases the stability of the NC0 complex bonded to the Rh. Recently, a similar result was obtained on Pt(lOO) by vibrational EELS measurements [24]. In this case it was possible to establish that preadsorbed oxygen caused a shift in v,,(NCO) towards higher frequency. This may indicate that the primary reason for the stabilizing effect of adsorbed oxygen is that, through a ligand effect, it results in a more ionic bond and thereby in the higher stability of Rh-NCO. As this observation may shed new light on the role of the NC0 complex in
F. Solymosi et al. / NC0 on Rhjl II) surfaces
245
the NO + CO reaction, it was important to determine whether this stabilizing effect of adsorbed oxygen holds under conditions more closely approaching those prevailing during the catalytic reduction of NO, i.e. elevated pressure and supported catalyst. The NC0 species was produced in this case too by the adsorption of HNCO at 200-300 K. The data obtained so far on silica-supported Pt and Rh [26] showed good agreement with those measured on single-crystal surfaces under UHV conditions.
References [I] [2] [3] [4] [5] [6] [7] [S] [9] [lo] (111 [12] [13] [14] [15] (161 [17] (181 [19] [20] [21] [22] [23] (241 [25] [26] [27]
F. Solymosi and J. Kiss, in: Proc. IVC-8, ICSS-4, ECOSS-3, Cannes, 1980. p. 213. F. Solymosi and J. Kiss, Surface Sci. 104 (1981) 181. R.j. Gorte, L.D. Schmidt and B.A. Sexton, J. Catalysis 67 (1981) 387. F. Solymosi and J. Kiss, Surface Sci. 108 (1981) 368. J. Kiss and F. Solymosi, Surface Sci. 135 (1983) 243. M.L. Unland, J. Catalysis 31 (1973) 459. H. Arai and H. Tominaga. J. Catalysis 43 (1976) 131. R. Nakamura, R. Nakai. K. Sugiyama and E. Echigoya, Bull. Chem. Sot. Japan 54 (1981) 1950. F. Solymosi and J. SPrkPny, Appl. Surface Sci. 3 (1976) 68. F. Solymosi, L. Volgyesi and J. Raskb, 2. Physik. Chem. (NF) 120 (1980) 79. W.C. Keller and A.T. Bell. J. Catalysis 84 (1983) 200. F. Solymosi and J. Rasko, unpublished results. J. Rasko, L. Volgyesi, M. Lancz and F. Solymosi, in: Proc. 8th Intern. Congr. on Catalysis, Berlin, 1984, in press. R.A. Ashby and R.L. Werner, J. Mol. Spectrosc. 18 (1965) 184. P.A. Thiel, J.T. Yates, Jr. and W.H. Weinberg, Surface Sci. 82 (1979) 45. F. Solymosi and J. Kiss, J. Catalysis 81 (1983) 95. A. Berko, Thesis, University of Szeged (1983). F. Solymosi, A. Berko and T.I. Tarn&xi, Surface Sci. 141 (1984) 533. D.G. Castner, B.A. Sexton and G.A. Somorjai, Surface Sci. 71 (1978) 519; P.A. Thiel, A.D. Williams, J.T. Yates and W.H. Weinberg, Surface Sci. 84 (1979) 54. J. Kiss and F. Solymosi, unpublished results. CT. Campbell, S.K. Shi and J.M. White, Appl. Surface Sci. 2 (1979) 382. L.H. Dubois and G.A. Somorjai, Surface Sci. 128 (1983) L231. J. Rasko and F. Solymosi, J. Catalysis 75 (1982) 78. M. Surman. F. Solymosi, P. Hoffmann and D.A. King, J. Catalysis, in press. J.J. Zinck and W.H. Weinberg, J. Vacuum Sci. Technol. 17 (1980) 188. F. Solymosi and J. Rasko. J. Appl. Catalysis 10 (1984) 19. G. Ertl, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis. Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) p. 82.