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A RAIRS and HREELS study of acetone on Pt(111)
Surface Science 254 (1991) I-11 North-Holland
A RAIRS and HREELS study of acetone on Pt( 111) M.A.
Vannice
‘. W. Erley
and
H. Ibach
Institut ftirGre~z~~chenforschung und Vakuu~~hysik, Forschungszentrum Jiiiich, Postfach 1913, W-51 70 Jiilich, Germuny Received
12 October
1990; accepted
for pub~cation
5 March
1991
The use of RAIRS (reflection-absorption infrared spectroscopy) has provided the first IR spectra of acetone adsorbed on an unsupported metal surface. These results were complemented by HREELS. Both sets of spectra gave peak positions for multilayer acetone that were very similar to those associated with liquid-phase acetone, whereas the v’ (“end-on” coordination) monolayer state had a C=O stretch frequency red-shifted to 1638 cm-‘, thus making it easy to identify. Direct evidence for an 1’ species, which has been proposed to exist in a “side-on” configuration with its C=O bond parallel to the surface, was not easy to obtain on this essentially defect-free Pt(ll1) surface because of the absence of higher-energy binding states that have been previously reported for Pt(1 II), Pd(ll1) and Ru(001) surfaces. However, at 1 K/s a second desorption peak at 199 K could be resolved from the larger nt peak at 184 K. These thermal desorption spectra also indicated that no acetone decomposition occurred during desorption. ‘Ibis more strongly bound acetone species is associated with an p* species, but the small binding energy difference (1 kcal/mol) appears to be Iess on this surface compared to the others and increases the difficulty of isolating this n* state. Additional evidence for a monolayer species other than the n’ phase was provided by RAIRS and HREELS. First, the TJ’ species that initially forms gradually disappears as the multilayer phase develops. Second, after flashing to temperatures below 200 K the 1638 cm-* band is removed, yet peaks at 1086, 1360, and 1428 cm-‘, associated with the methyl groups, not only are retained but also grow in intensity, and new bands appear between 1500 and 1610 cm-‘. Finally, these latter bands are also formed after the dissociative adsorption of isopropyl alcohol to give adsorbed hydrogen and acetone.
1. Introduction Acetone adsorbed on Pt has not been widely characterized. An initial EELS study by Avery reported spectra for both multilayer and monolayer states on the Pt(ll1) surface [I]. One monolayer state (77’) provided a well-defined EEL spectrum and thermal desorption (TD) peak distinguishable from those associated with the acetone multilayer, whereas a second state (q2) was inferred from TD spectra although no EEL spectrum was reported for this species. The qt(O)(CH,),CO state ($) has been assigned to an acetone species bonding end-on via the lone pair electrons on the oxygen, while the ~~(0, C)(CH,),CO state (?t2) was proposed to involve a
’ To whom inquiries should be sent. Current address: Pennsylvania State University, Department of Chemical gineering, University Park, PA 16802, USA.
0039-6028/91/$03.50
0 1991 - Elsevier Science Publishers
The En-
species adsorbed with its C=O bond parallel to the surface, giving either r-bonding or di-a bonding after rehybridization [l]. The possible importance of differences in the reactivity of these two species has been discussed in a recent study of acetone hydrogenation over Pt by Sen and Vannice in which 400-fold enhancements in turnover frequency with certain supports were attributed to the presence of sites favoring an n2 configuration
PI. To gain additional insight into the adsorption states of acetone on Pt and to try to resolve spectra of the elusive q2 species, we utilized reflection-absorption IR spectroscopy @AIRS), highresolution electron energy loss spectroscopy (HREELS) and TDS to characterize acetone adsorbed on a clean Pt(ll1) surface as well as acetone coadsorbed with hydrogen on this surface. The higher resolution of the IR technique clarified the different vibrational bands while HREELS, which
B.V. (North-Holland)
M.A. Vannice et al. / A RAIRS
2
and HREELS
gave overlapping bands in the higher wavenumber region, provided characterization in the lower wavenumber region which was not accessible with our IR detector. The IR characterization of acetone adsorbed on a metal surface has not been previously reported. These complementary results are presented here.
2. Experimental The UHV system containing both RAIRS and HREELS, along with LEED, AES, QMS and an ion gun, has been described in detail previously [3]. Computer-controlled thermal desorption capabilities have subsequently been added. The Pt(ll1) single crystal was oriented within O.l” and polished by a special procedure that gives essentially defect-free surfaces [4]. The sample could be heated either by radiation or electron bombardment from a tungsten filament mounted at the rear of it and could be cooled by liquid nitrogen to 85 K. The temperature was measured by a chromel-alumel thermocouple spot-welded
Acetone
on Pt 1111)
study
of acetone on Pt(lll)
on the front edge of the sample. The pre-cleaning of the Pt(ll1) surface was done by an extended bombardment with Ne ions at 300 K. After a flash of the Pt sample to 1500 K and retooling to room temperature, the LEED observation of sharp (1 x 1) spots indicated a proper recrystallization of the surface. The small amount of carbidic carbon which always remained on the surface after sputtering with Ne ions could be effectively removed by heating the sample to 900 K for 5 min in an ambient oxygen pressure of 10e6 Torr. After a flash to 1250 K and subsequent cooling to 85 K, no surface impurities were detectable by AES above the noise level. Acetone (Merck, spec. grade) or completely deuterated acetone (MSD, 99.5 at% d,-acetone) was introduced from a small glass bulb via an adjustable leak valve. As the latter was mounted at a distance of about 80 mm directly in front of the sample, the local pressure at the sample surface was always higher than that measured by the ion pressure gauge in the main chamber. All exposures which are given in the text and in the figures are corrected values which have been estimated by
A
B II
Acetone
on Pt (Ill)
a
A-----z xx)
209
300
TEMPERATURE
LOO
500
I
IK 1
Fig. 1. (A) Thermal desorption spectra (3 K/s) of acetone adsorbed on Pt(ll1): (a) after an exposure of 0.8 L, (b) after an exposure of 0.4 L, (c) after an exposure of 0.4 L of de-acetone at 85 K, (d) after an exposure of 0.4 L acetone to a surface precovered with hydrogen (0.25 monolayer). (B) Thermal desorption spectra (1 K/s) following acetone adsorption (1.6 L exposure) on Pt(ll1): (a) hydrogen (mass 2) (b) CO (mass 28) (c) acetone (mass 43).
MA. Vannice et al. / A RAIRS and HREELS study of acetone on Pt(lllJ
comparing a few infrared spectra to those obtained by backfilling the chamber with acetone. All exposures were at 85 K unless otherwise noted. Infrared spectra with 4 cm-’ resolution were taken by averaging 660 scans over 4 min. As the MCT infrared detector has a cut-off frequency of 800 cm-‘, HREELS (AE = 40 cm-“) was used in addition, mainly to get spectroscopic information in the low frequency range. For the TD spectra a heating rate of either 3 or 1 K/s was used, as stated in the figures. All RAIRS and HREELS data were obtained at 85 K unless otherwise noted.
f
*
Acetone
on
3 Pt (III)
3. Results Thermal desorption (TD) spectra were obtained for acetone on the clean Pt(ll1) surface as well as on one preexposed to 3 L H, at 90 K. The latter surface represents a hydrogen coverage of about 0.25 according to the work of Christmann and Ertl [S]. For acetone the major fragmentation peak at 43 amu (46 amu for acetone-d,) was monitored. Fig. 1A shows the TD spectra for multilayer and monolayer coverages of acetone, for near-monolayer coverage of completely deuterated acetone, and for acetone coadsorbed with hydrogen using a heating rate of 3 K/s. The multilayer phase desorbs near 149 K while the monolayer state desorbs near 199 K. The presence of hydrogen on the surface lowers these peaks by about 9 K to 140 and 189 K, respectively. The principal monolayer peak at 185 K for d, acetone is lower by about 13 K. To improve the resolution of more than 1 desorption peak near 200 K, a heating rate of 1 K/s was also used. In addition, to determine if any H, or CO was desorbing, masses at 2 and 28 amu were monitored as well as mass 43, and these spectra are shown in fig. 1B. At this heating rate well-resolved peaks at 130, 184 and 199 K were obtained, and little or no decomposition to CO and hydrogen was observed. The TD spectra are raw, unsmoothed data obtained with a feedback loop which provided a linear heating rate, and some perturbations were sometimes observed during the initial startup period because of this feedback loop. These baseline fluctuations in the lowest temperature region could
1 %,
r
I,
I
,“,I,
1,
zoo0
I
looo WAVENUMBER
b-i’)
Fig. 2. RAIR spectra at 85 K of acetone adsorbed on Pt(ll1): (a) after an exposure of 1 L (7’ i-multilayer species), (b) after an exposure of 2 L, (c) after an exposure of 3 L, (d) after an exposure of 4 L (multilayer species), (e) after a flash to 120 K of the preceding surface, (f) after a second flash to 140 K of the preceding surface.
produce shoulders at times, such as in spectrum a, fig. 1A. Infrared spectra were obtained after increasing acetone exposures at 85 K. For acetone on a clean Pt(l11) surface following the final exposure of 4 L, the crystal was flashed to 120 K (10 K/s) and an IR spectrum was obtained, then it was flashed to 140 K and a final spectrum was run. This series of IR results is shown in fig. 2. The peaks in fig. 2 spectra b, c and d are near those for liquid-phase acetone [6,7] and easily associated with multilayer acetone [l]. To better resolve the vi monolayer phase, which is associated with the C=O stretching frequency at 1638 cm-’ [l], a lower exposure of 0.4 L was used. The sample was again flashed to 140 K after this measurement and these two spectra are provided in fig. 3. The high sensitivities required to resolve IR peaks for submonolayer coverages of acetone should be noted; for exam-
4
M.A. Vannice et al. / A RAIRS
and HREELS
ple, the 2060 cm-’ peak for CO in fig. 3, spectrum b, represents a coverage of approximately 0.001 monolayer of CO. To examine the possibility that the $ species can convert to the n* species at temperatures near the desorption temperature, the crystal was held at 170 K to prevent multilayer formation but retain the monolayer species, acetone was adsorbed, and a spectrum was obtained at this temperature. The crystal was warmed to 180 K and then to 200 K with an IR spectrum obtained at each temperature. These three spectra are shown in fig. 4. As anticipated, the n’ phase was the principal one present at the two lower temperatures, but the peak at 1646 cm-’ for the C=O stretch had disappeared at 200 K although new bands in the carbonyl region at 1612,1584, and 1511 cm-’ had grown in and bands associated with the methyl groups still remained. Spectra for deuterated acetone, (CD,),CO, were obtained under conditions identical to those used
I
Acetone
on Pt (111)
WAVENUMEIER hi’ 1 Fig. 3. RAIR spectra at 85 K of acetone adsorbed on Pt(ll1): (a) after an exposure of 0.4 L ( TJ’ species), (b) after flashing the preceding surface to 140 K (7’ species).
study of acetone on Pt(1 I I)
Acetone
2oal
on Pt (111)
loo0
WAVENUMBER km-‘) Fig. 4. RAIR spectra of acetone adsorption on Pt(ll1) at higher temperature: (a) at 170 K following an exposure of 0.4 L at 170 K (primarily 9’ species), (b) at 180 K after warming the preceding surface, (c) at 200 K after further warming of the preceding surface ($ species).
for (CH,),CO. The increase in band intensities with increasing exposure at 85 K on Pt(ll1) is shown in fig. 5 along with the state of the surface after a flash to 170 K following the last cumulative exposure of 0.32 L. After a 0.4 L exposure only the monolayer state was present, but as previously observed, as the multilayer peak (1710 cm-‘) grew, the 1626 cm-’ peak associated with the n’ species decreased significantly. The sensitivity and resolution capability of the RAIRS technique is particularly apparent in this series of runs as the various deformation and rocking modes of the -CD, groups are nicely resolved in the 9501050 cm-’ region. Because of the interest in the reaction between acetone and H, to form isopropanol [2], an approximate hydrogen coverage of 0.25 monolayer was established and acetone was adsorbed on this surface. An exposure of 0.4 L gave the monolayer state with only a small peak at 1712 cm- I, which
M.A. Vannice et al. / A RA IRS and HREELS
study of acetone on Pt(llIf
is
representative of the C=O stretch frequency in a multilayer phase. A further cumulative exposure to 0.8 L increased the intensity of the 1712 cm-’ peak and reduced the intensity of the 1640 cm-’ peak, as shown in fig. 6. This concomitant decrease in the 1640 cm-’ peak as the 1712 cm-’ peak increased thus reproduced the trend in figs. 2 and 5 with acetone alone on a clean Pt surface. HREEL spectra were taken after exposures identical to those used for the IR results. Spectra a and d in fig. 7 show the EELS bands present after 0.4 and 3 L exposures, respectively. After each of these spectra was taken, the surface was flashed to 170 K and another spectrum was obtained, and these are presented in fig. 7, spectra b and e. Finally, the EEL spectrum shown in fig. 7, spectrum c, was taken for the acetone coadsorbed with hydrogen immediately after the IR spectrum in fig. 4, spectrum a, was obtained. EEL spectra were
Acetone
5
on t-i-covered
Pt (111)
a
b
1”
s r
I’
‘I1
2m WAVENUMBER Acetone
on Pt (111)
b
I” ?axl
(cm-‘)
Fig. 6. RAIR spectra at 85 K of acetone coadsorbed with hydrogen on Pt(ll1): (a) after exposure of a surface precovered with 0.25 monolayer hydrogen to 0.4 L acetone, (b) after exposure of the preceding surface to an additional 0.4 L acetone.
also acquired for deuterated acetone after exposures of 0.4 and 0.8 L and then after a flash to 170 K following the final exposure. These spectra are given in fig. 8. In these two figures, the specular peak heights represent count rates between 4 x lo4 and 2.3 X lo5 counts per second, as noted in the figure captions.
I
‘2doo’
.
*
’
I
WAVENU~BER Fig. 5. RAIR spectra at 85 after an exposure of 0.4 L (q’ 0.8 L, (c) after an exposure preceding surface
c
I
t
d
:3
v*.
kx-i’J
K of d,-acetone on Pt(ll1): (a) species), (b) after an exposure of of 1.6 L, (d) after flashing the to 170 K (q2 species).
4. Discussion The only study of acetone adsorbed on Pt reported in the literature is that of Avery, who employed TDS and EELS to identify different adsorbed states on a Pt(ll1) surface [l]. Two species were proposed to exist up to monolayer coverage - an $ species bonded end-on through the oxygen atom and an q* species adsorbed with the C=O bond horizontal to the surface in either a a-bonded or d&-bonded configuration. Similar
6
MA.
Vannice et al. / A RAIRS
and HREELS
study
species have subsequently been proposed to exist on the Ru(001) and Pd(ll1) surfaces [8,9]. Whereas multilayer acetone on Pt(ll1) desorbed at 137 K, the 7’ species desorbed at 185 K and the n2 species was proposed to desorb over a range of
Acetone
of acetone on Pt(lI1)
.,ooo Acetone-d,
on Pt (111)
on Pt (111)
JL
I
0
m WAVENUMBER
2030 [cm-‘)
Fig. 8. HREEL spectra at 85 K of de-acetone [(CD,),CO]: (a) after an exposure of 0.4 L (TJ’ species) (4.2 x lo4 cps), (b) after an exposure of 0.8 L (9.8~10~ cps), (c) after flashing the preceding surface to 170 K (4.1 x lo4 cps).
t:
wjkky 3cKm WAVENUMBER
!I
km-‘)
Fig. 7. HREEL spectra at 85 K of acetone adsorbed on Pt(ll1); (the number in parentheses for each spectrum represents the specular peak height in counts per second): (a) after an exposure of 0.4 L (r$ species) (1.7 ~10~ cps), (b) after flashing the preceding surface to 170 K (9.5 X lo4 cps), (c) after exposure of a surface precovered with 0.25 monolayer hydrogen to 0.4 L acetone (4.3 x lo4 cps), (d) after an exposure of 3 L (multilayer species) (2.2~ lo5 cps), (e) after flashing the preceding surface to 170 K (2.3 X lo5 cps).
temperatures above 200 K based on rather unusual TD spectra [l]. In the TD spectra in fig. 1, the peaks for the multilayer and monolayer phases are clearly resolved by well-behaved TD curves, with the peak temperatures increased by 16-18 K at the higher heating rate. At the heating rate used by Avery (1 K/s), our desorption peak at 184 K for the n’ species nicely reproduces his value of 185 K, and the multilayer peak at 130 K is near his value of 137 K. These peaks are relatively sharp, and at the lower heating rate an additional peak at 200 K can be resolved. Unlike previous TD spectra for acetone on Pt(ll1) [l], Ru(001) [8], and Pd(lll) [9], the spectra in fig. 1 exhibit no tailing and thus indicate the absence of higher-energy binding
1370 1235 1105 910
1373 1239 1094
900
1360 1220 1090 1065 900 783 530 484 390
WW
a) Me designates a methyl group. b, From refs. [5,6]. ‘) From ref. (131.
WO) r(CO) G(MeCMe)
v,(MeCMe)
P We)
P(Me)
v,(MeCMe)
405
550
1430
1443 1422
1445 1428
WW
1350 1238 1086
1426
810 570
1355 1250 1080
1440
1640
1638
2920 1715
1716
3005
3010
3005 2960 2925 1712
v(W
$-acetone
1365 1240 1086
1610 1530-1585 1511 1420-1445
IR
EELS
From IPA ‘)
830
1550
1360-1410 1262 1085
1611 1584 1512 1460 1004
IR
1085 1245 960 887 724 690 480 405 326
1035
2255 2222 2120 1700
Liquid IR b,
IR EELS
q’-acetone
EELS
IR
Liquid IR b,
Multilayer
(CD,),CO
(CH,)aCO
for acetone on Pt(ll1)
We)
Mode ‘)
Vibrational mode assignments (cm-‘)
Table 1
1090 1261 960
1036
1710
IR
1690
EELS
Multilayer
1280 958
1040 1025 992 983
1626
IR
850 715 520
1275
985
1030
1620
2230
EELS
+-acetone
IR
1262 959
959
1039
1611 1512
$-acetone
a’: 3 3. w
% ?-
8
M.A. Vannice et al. / A RA IRS and HREELS
states (extending up to 250 K on Pt and 400 K on Ru and Pd) that have been as>ociated with the q2 species on these surfaces. The absence of any binding states above 220 K could be a consequence of a cleaner surface or, more likely, a much lower concentration of defect sites, as the n2 species has been proposed to adsorb on step sites on the Pt(ll1) surface [l]. The results in fig. 1 also imply that if the 9’ state can convert to the n2 species, it does so in the same temperature range that the desorption process occurs thus making it much more difficult to isolate the n2 species on Pt(ll1) compared to the Ru(001) and Pd(ll1) surfaces. Using the Redhead equation with v = 10” s-’ [lo], desorption energies of 12.5, 11.5, and 8.0 kcal/mol can be calculated from fig. 1B for the n2, n’, and multilayer acetone species, respectively. The last value is very close to the heat of vaporization of 7.64 kcal/mol for acetone [ll]. This small difference in binding energy between the 7’ and n2 states is one reason why it is difficult to form and isolate the latter species on Pt(ll1). Little or no decomposition of acetone occurred on this surface, as indicated by the absence of a CO peak near 460 K [12] and any significant H, peak, and this may again be due to the absence of defect sites. In other TDS runs showing the desorption of acetone and H, formed by the decomposition of isopropanol on this surface, no CH, was observed [13]; consequently, we do not expect any CH, formation here. The IR spectra for both the monolayer and multilayer states give peak positions in excellent agreement with the assignments made by Avery [l]. As illustrated in table 1, the IR peaks for the multilayer state agree closely with the bands reported for liquid-phase acetone [6,7]. The multilayer HREELS peaks are also included and, although their resolution is lower, they are in excellent agreement with the IR peaks and correspond well to the liquid-phase positions. There is again a one-to-one correspondence between the IR and EELS peaks that are associated with the 17’ state, and they also agree extremely well with those reported by Avery. The C=O stretch frequency exhibit a red shift of about 75 cm-‘, while smaller blue shifts (15-30 cm-‘) occur for the C-C-C
study of acetone on Pt(l II)
skeletal modes, and the methyl groups are essentially not affected. The same trends exist for the deuterated 7’ acetone species, whose peak positions are also listed in table 1. All these assignments have been discussed in detail [1,8,9]. Our principal intent in this study was to examine the IR spectra as well as the HREEL spectra for evidence substantiating the presence of an n2 species. An intriguing pattern was always observed with increasing exposure to acetone. A comparison of spectrum 3a with those in fig. 2 shows that the development of the multilayer state results in the disappearance of the n* state because the 1638 and 1350 cm-’ bands associated with the latter disappear and are replaced by the 1716 and 1373 cm-’ multilayer peaks. High exposures are not required for this transformation; for example, as shown in fig. 5, spectrum c, an exposure of less than 2 L noticeably suppresses the CO band in the n’ (CD,),CO species (1626 cm-‘) while the same band for the multilayer species at 1710 cm-’ grows significantly. Two explanations can account for this curious behavior: either the 9’ species interacts more strongly with the polar acetone molecules in the second layer than with the Pt surface, thus removing the end-on bonding alignment with the Pt surface, or a restructuring of the 71’ phase occurs to create a monolayer phase with the C=O bond parallel to the surface thus reducing the normal component of the dipole moment and making it more difficult to detect. The latter possibility would produce an n* species, and it may be more likely because the desorption energies indicate a stronger interaction between acetone and the Pt surface than between acetone molecules. The IR spectra for the monolayer state of acetone on Pt(ll1) repeatedly exhibited peaks between 1500 and 1610 cm-’ that were difficult to resolve with HREELS because of their low intensity. One example is provided by a comparison of fig. 3, spectrum a, in which bands at 1609 and 1520 cm-’ can be seen, with fig. 7, spectrum a, in which these two bands are not resolved. It is possible that these weak peaks could represent n2 species in different configurations [1,X]. To determine if conversion from the n’ to the n2 species could occur at higher temperatures, as reported
M.A. Vannice et al. / A RA IRS and HREELS
for the Pd(ll1) surface [9], the monolayer coverage in fig. 3, spectrum a, was flashed to 140 K and re-examined. Spectrum 3b shows that the 1638 yet bands between cm ’ band had disappeared, 1550 and 1610 cm-’ remained while the 1429 and 1086 cm-’ peaks associated with deformation and rocking modes of the methyl groups had grown substantially. The retention of position for the methyl peaks at 1086, 1360 and 1428 cm-’ establishes that an intact acetone species remains, but it has no detectable peak for the C=O stretch at 1638 cm-‘. In addition, the intensities of these three bands increase rather significantly as the 1640 cm-’ peak vanishes. This same trend can be seen in the HREELS spectra in fig. 7, spectra a and b. Although not all the 7’ species was removed, the 1640 cm-’ peak diminished noticeably while the bands at 830, 1085, and 1420 cm-’ increased in intensity. Also, a 1550 cm-’ peak was resolved in spectrum 7b. The intensities of the peaks associated with the methyl groups after flashing to 140 or 170 K indicate that rather high surface coverages remain and, in fact, the appearance of the 1711 cm-’ peak in fig. 3, spectrum b, for multilayer acetone implies that the surface must have nearly saturated with species constituting monolayer coverage. During the rapid flash some CO formation occurred to give the bands at 2050 and 1806 cm-‘, which are associated with CO adsorbed on on-top and 3-fold hollow sites, respectively [14]. This decomposition is attributed to the hot tungsten wire wrapped around the edge of the Pt crystal and used for heating rather than to the Pt surface itself. For example, no CO was observed in fig. 1B when a much lower heating rate was used. The band near 1610 cm-’ and those between 1520 and 1680 cm ’ cannot be associated with isopropanol [13] and are almost certainly associated with carbonyl bonds, thus they may possibly represent more strongly coordinated n2 species. Further support for this assignment comes from the IR spectra in fig. 4, which represent acetone adsorbed at 170 K, rather than at 85 K, and subsequently warmed gently to 200 K. The initial state at 170 K gives a spectrum quite similar to that of fig. 3, spectrum a, with bands near 1610 and 1500 cm-’ being distinguishable along with those expected
study of acetone on Pt(l I I)
9
for the $ state. Warming to 200 K removed the 1646 cm-’ peak but increased the band intensities at 1610, 1584, and 1511 cm-‘, while bands near 1425 and 1360 cm-‘, associated with the methyl groups, remained. The growth of the 2070 cm-’ peak again indicates that some decomposition to CO was occurring but again this is attributed to the hot tungsten wire. The peak at 2080 cm-’ in fig. 4, spectrum c, represents a surface coverage of about 0.1 monolayer CO. The possibility that the three peaks between 1500 and 1610 cm-’ represent CO adsorbed on high-coordination sites cannot be discounted; for example, very weak peaks at 1410 and 1650 cm-’ have been reported for CO on a Pt 6(111) X (111) surface at low coverage [12]. However, at this time we do not think these peaks represent molecular CO species for the following reasons. First, these low-coordination sites were not observed in a HREELS study of CO on Pt(ll1) [12]. Second, RAIR spectra of CO adsorbed on our Pt(ll1) surface showed no peaks in this region. Third, the intensities of the bands between 1500 and 1610 cm-’ do not correlate with the intensity of the CO band near 2070 cm-‘, as shown by a comparison of figs. 2 (spectrum a), 3 (a and b) and 6 (b). Finally, the decomposition of isopropanol on this crystal surface also produced peaks at 1611, 1584, and 1512 cm-’ without the formation of CO [13]. Consequently, these bands may be representative of n2 species. The IR spectra in fig. 5 for d,-acetone showed well-resolved bands in the 950-1050 cm-’ region that are associated with the deformation and rocking modes of the -CD, groups. Again the growth of the multilayer peak at 1710 cm-’ suppressed the 1625 cm-’ peak of the monolayer species, and the C-C-C skeletal mode at 1261 cm-’ of the multilayer deuterated species became especially intense. Flashing to 170 K left well-defined peaks at 1039 and 959 cm-’ that were stronger than those at monolayer coverage (fig. 5, spectrum a) along with a band at 1262 cm-’ indicating that intact acetone species were still present. Peaks at 1611 and 1512 cm-’ were again observed. The peaks in the 1360-1430 cm-i region are attributed to a small amount of nondeuterated acetone displaced from the walls of the chamber by d,acetone and readsorbed on the Pt surface.
10
MA. Vannice et al. / A RAIRS and HREELS study of acetone on Pt(l li)
To examine the influence of coadsorbed hydrogen, acetone was adsorbed on a surface precovered with 0.25 monolayer hydrogen. The spectrum after a 0.4 L exposure (fig. 6, spectrum a) is similar to that of acetone on the clean surface after the same exposure; however, the adsorption capacity for the 7’ species was somewhat reduced because the 1640 cm-’ band on the H-covered surface was less intense as the multilayer 1712 cm-” band began to form. An exposure of 0.8 L gave both mono- and multilayer species, as shown in fig. 6, spectrum b, but a distinct peak near 1530 cm -I can also be observed. A similar set of experiments was conducted with HREELS on two surfaces: one precovered with 0.25 monolayer of hydrogen and the other precovered with 0.5 monolayer. The HREEL spectrum for acetone on the former H-covered surface is shown in fig. 7, spectrum c, and it has particuiarly good peak resolution. Although there are some small shifts in the peak positions, there is a one-to-one correspondence between the IR and HREEL peaks above 1000 cm - ’ and the rf species can easily be identified. The HRBEL signal was much weaker for acetone adsorbed on the surface coveted with 0.5 monolayer H, thus indicating that the surface coverage was substantially lower and that hydrogen and acetone compete for the same sites, as indicated by kinetic models for acetone hydrogenation on Pt [2]. Regardless, bands at 1620, 1410, 1070, 830, and 560 cm-’ could be clearly resolved. After flashing this latter surface to 170 K, the 1620 cm-’ peak diminished, the 1410, 820, and 560 cm-’ peaks were relatively unchanged, the 1080 and 910 peaks associated with methyl group rocking modes and the 410 cm-’ peak associated with a skeletal deformation mode intensified, and a weak band near 1550 cm-’ could be identified. The peaks that have been observed for adsorbed monolayer acetone in the absence of the $ species (i.e., no 1640 cm-’ peak), and which could therefore represent bands associated with nz species, are summarized in table 1, They have been obtained primarily from figs. 3 (spectrum b), 4 (c), 5 (d) and 6 (b). The principal difference between these peaks and those found for the T.# species are the carbonyl bands near 1511, 1550,
and 1610 cm-‘. These are tentatively assigned to adsorbed acetone species more strongly coordinated to the surface than the 9’ species, and whose carbonyl stretch mode is much more difficult to detect, thus these bands may represent the n2 species proposed by Avery [l]. These carbonyl bands do have higher frequencies than those assigned to n2 species on Ru(001) by Anton et al. [8] and on Pd(ll1) by Davis and Barteau [9]; however, Lercher et al. have associated bands at 1580 and 1520 cm-l with a “side-on” configuration for acetone adsorbed on MgO [16]. In the study of Anton et al., the 1275 cm-’ peak for de-acetone, which was associated with the C=O stretch frequency, corresponds to the C-.C.-C skeletal stretch frequency of 1280 cm-’ on Pt(l11) in fig. 5 and it is near the bands of 1260-1300 cm-r on Pd(ll1) [9]. Since the carbonyl band stretch intensity of any species with the C=O bond parallel to the surface is expected to be weak [15], it is possible that the relative growth of this band on Ru as the carbonyl band at 1665 cm-’ disappears is a consequence of an enhanced contribution from the skeletal mode. Regardless, it is still indicative of an n2 species. The CO stretch frequency near 1440 cm-’ assigned to an n2 species on Pd(ll1) [9] is not too much lower than the bands observed on our Pt(ll1) crystal #I. In conclusion, there is evidence, although it is not easily obtained, that an adsorbed acetone species exists on a Pt(ll1) surface with its C=O bond oriented such that it is difficult to detect with vibrational spectroscopy. Thus the conclusion by Avery {l] that such an n2 species occurs on Pt is supported by this study. The evidence consists of: (1) a TD peak at 199 K attributed to an n2 species in addition to that at 184 K for the n’ state, (2) the disappearance of the 1640 cm- ’ peak for the T# species as the multilayer phase forms, (3) the retention and enhancement in methyl peak intensities in the absence of the 1640 cm -I band,
One reviewer raised the possibility that bands between 1500 and 1600 cm’-’ may represent acetyl (CH,C=O) species; however, McCabe et at. have associated bands at 610, 900-925, 1120 and 1650 cm-’ with this species on a Pt(S~-[6(lll)X(l~)~ surface 1171, thus we do not believe this species is present.
M.A. Vannice et al. / A RA IRS and HREELS
(4) the appearance of weaker bands between 1500 and 1610 cm-’ as the 1640 cm- ’ band decreases or disappears, and (5) the generation of these same lower-frequency bands during the decomposition of isopropyl alcohol to hydrogen and acetone [13].
5. Summary The use of RAIRS has provided the first IR spectra of acetone adsorbed on an unsupported metal surface. These results were complemented by HREELS to cover the frequency range between 200 and 3500 cm-‘. The RAIR spectra readily resolved all the peaks expected from liquid-phase IR spectra and gave very similar peak positions for all the multilayer acetone bands whereas the 17’ monolayer state had a C=O stretch frequency red-shifted to 1638 cm-‘. Direct evidence for an nz species, proposed to exist with its C=O bond parallel to the surface, was not easy to obtain with our essentially defect-free Pt(ll1) surface because of the small (1 kcal/mol) difference in binding energies between the TJ~ and q2 states and the absence of higher-energy binding states for the n2 species that have been reported for other metal surfaces. However, a second desorption peak at 199 K could be resolved from the T/ peak at 184 K, and this more strongly bound acetone species is associated with an n2 state. TDS also showed that no acetone decomposition occurred during desorption. Additional evidence for a monolayer species other than the 7’ phase was provided by the RAIRS and HREELS results. First, the n1 species that initially forms gradually disappears as the multilayer phase develops. Second, after flashing to temperatures below 200 K the 1638 cm-’ band is absent yet peaks at 1086, 1360 and 1428 cm-‘, associated with the methyl groups, not only are retained but also grow in intensity. Furthermore, new bands between 1500 and 1610 cm-’ appear.
study of acetone on Pt(l I I)
11
Finally, these latter bands have also been observed after the dissociative adsorption of isopropyl alcohol to give hydrogen and acetone [13].
Acknowledgements One of us (M.A.V.) would like to thank not only the Alexander von Humboldt-Stiftung for a Senior Scientist Research Award during the period this work was conducted, but also the IGV Forschungszentrum Jtilich for making his stay there possible. The assistance of C. Damarow in the laboratory was also greatly appreciated.
References 111N.R. Avery, Surf. Sci. 125 (1983) 771. 121B. Sen and M.A. Vanmce, J. Catal. 113 (1988) 52. [31 W. Erley, J. Electron. Spectrosc. Relat. Phenom. 44 (1988) 65. [41 U. Linke and B. Poelsema, J. Phys. E (Sci. Instrum.) 18 (1985) 26. [51 K. Christmann and G. Ertl, Surf. Sci. 60 (1976) 365. 161 T. Shimanouchi, Tables of Molecular Vibrational Fre39 (US quencies, Consolidated Vol. I, NSRDS-NBS Government Printing Office, Washington, 1972). Atlas of 171 B. Schrader and W. Meier, Eds., Raman/IR Organic Compounds, Vol. 1 (Verlag Chemie, Weinheim, 1977). PI A.B. Anton, N.R. Avery, B.H. Toby and W.H. Weinberg, J. Am. Chem. Sot. 108 (1986) 684. 191 J.L. Davis and M.A. Barteau, Surf. Sci. 208 (1989) 383. WI P.A. Redhead, Vacuum 12 (1962) 203. [111 R.C. Weast, Ed., CRC Handbook of Chemistry and Physics, 59th ed. (CRC Press, Boca Raton, 1978). WI H. Hopster and H. Ibach, Surf. Sci. 77 (1978) 109. 1131 M.A. Vanmce, W. Erley and H. Ibach, Surf. Sci. 254 (1991) 12. 1141 B.E. Hayden and A.M. Bradshaw, Surf. Sci. 125 (1983) 787. P51 H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982). iI61 J.A. Lercher, H. Noller and G. Ritter, J. Chem. Sot. Faraday Trans. I, 77 (1981) 621. P71 R.W. McCabe, CL. DiMaggio and R.J. Madix, J. Phys. Chem. 89 (1985) 854.
A RAIRS and HREELS study of acetone on Pt( 111) M.A.
Vannice
‘. W. Erley
and
H. Ibach
Institut ftirGre~z~~chenforschung und Vakuu~~hysik, Forschungszentrum Jiiiich, Postfach 1913, W-51 70 Jiilich, Germuny Received
12 October
1990; accepted
for pub~cation
5 March
1991
The use of RAIRS (reflection-absorption infrared spectroscopy) has provided the first IR spectra of acetone adsorbed on an unsupported metal surface. These results were complemented by HREELS. Both sets of spectra gave peak positions for multilayer acetone that were very similar to those associated with liquid-phase acetone, whereas the v’ (“end-on” coordination) monolayer state had a C=O stretch frequency red-shifted to 1638 cm-‘, thus making it easy to identify. Direct evidence for an 1’ species, which has been proposed to exist in a “side-on” configuration with its C=O bond parallel to the surface, was not easy to obtain on this essentially defect-free Pt(ll1) surface because of the absence of higher-energy binding states that have been previously reported for Pt(1 II), Pd(ll1) and Ru(001) surfaces. However, at 1 K/s a second desorption peak at 199 K could be resolved from the larger nt peak at 184 K. These thermal desorption spectra also indicated that no acetone decomposition occurred during desorption. ‘Ibis more strongly bound acetone species is associated with an p* species, but the small binding energy difference (1 kcal/mol) appears to be Iess on this surface compared to the others and increases the difficulty of isolating this n* state. Additional evidence for a monolayer species other than the n’ phase was provided by RAIRS and HREELS. First, the TJ’ species that initially forms gradually disappears as the multilayer phase develops. Second, after flashing to temperatures below 200 K the 1638 cm-* band is removed, yet peaks at 1086, 1360, and 1428 cm-‘, associated with the methyl groups, not only are retained but also grow in intensity, and new bands appear between 1500 and 1610 cm-‘. Finally, these latter bands are also formed after the dissociative adsorption of isopropyl alcohol to give adsorbed hydrogen and acetone.
1. Introduction Acetone adsorbed on Pt has not been widely characterized. An initial EELS study by Avery reported spectra for both multilayer and monolayer states on the Pt(ll1) surface [I]. One monolayer state (77’) provided a well-defined EEL spectrum and thermal desorption (TD) peak distinguishable from those associated with the acetone multilayer, whereas a second state (q2) was inferred from TD spectra although no EEL spectrum was reported for this species. The qt(O)(CH,),CO state ($) has been assigned to an acetone species bonding end-on via the lone pair electrons on the oxygen, while the ~~(0, C)(CH,),CO state (?t2) was proposed to involve a
’ To whom inquiries should be sent. Current address: Pennsylvania State University, Department of Chemical gineering, University Park, PA 16802, USA.
0039-6028/91/$03.50
0 1991 - Elsevier Science Publishers
The En-
species adsorbed with its C=O bond parallel to the surface, giving either r-bonding or di-a bonding after rehybridization [l]. The possible importance of differences in the reactivity of these two species has been discussed in a recent study of acetone hydrogenation over Pt by Sen and Vannice in which 400-fold enhancements in turnover frequency with certain supports were attributed to the presence of sites favoring an n2 configuration
PI. To gain additional insight into the adsorption states of acetone on Pt and to try to resolve spectra of the elusive q2 species, we utilized reflection-absorption IR spectroscopy @AIRS), highresolution electron energy loss spectroscopy (HREELS) and TDS to characterize acetone adsorbed on a clean Pt(ll1) surface as well as acetone coadsorbed with hydrogen on this surface. The higher resolution of the IR technique clarified the different vibrational bands while HREELS, which
B.V. (North-Holland)
M.A. Vannice et al. / A RAIRS
2
and HREELS
gave overlapping bands in the higher wavenumber region, provided characterization in the lower wavenumber region which was not accessible with our IR detector. The IR characterization of acetone adsorbed on a metal surface has not been previously reported. These complementary results are presented here.
2. Experimental The UHV system containing both RAIRS and HREELS, along with LEED, AES, QMS and an ion gun, has been described in detail previously [3]. Computer-controlled thermal desorption capabilities have subsequently been added. The Pt(ll1) single crystal was oriented within O.l” and polished by a special procedure that gives essentially defect-free surfaces [4]. The sample could be heated either by radiation or electron bombardment from a tungsten filament mounted at the rear of it and could be cooled by liquid nitrogen to 85 K. The temperature was measured by a chromel-alumel thermocouple spot-welded
Acetone
on Pt 1111)
study
of acetone on Pt(lll)
on the front edge of the sample. The pre-cleaning of the Pt(ll1) surface was done by an extended bombardment with Ne ions at 300 K. After a flash of the Pt sample to 1500 K and retooling to room temperature, the LEED observation of sharp (1 x 1) spots indicated a proper recrystallization of the surface. The small amount of carbidic carbon which always remained on the surface after sputtering with Ne ions could be effectively removed by heating the sample to 900 K for 5 min in an ambient oxygen pressure of 10e6 Torr. After a flash to 1250 K and subsequent cooling to 85 K, no surface impurities were detectable by AES above the noise level. Acetone (Merck, spec. grade) or completely deuterated acetone (MSD, 99.5 at% d,-acetone) was introduced from a small glass bulb via an adjustable leak valve. As the latter was mounted at a distance of about 80 mm directly in front of the sample, the local pressure at the sample surface was always higher than that measured by the ion pressure gauge in the main chamber. All exposures which are given in the text and in the figures are corrected values which have been estimated by
A
B II
Acetone
on Pt (Ill)
a
A-----z xx)
209
300
TEMPERATURE
LOO
500
I
IK 1
Fig. 1. (A) Thermal desorption spectra (3 K/s) of acetone adsorbed on Pt(ll1): (a) after an exposure of 0.8 L, (b) after an exposure of 0.4 L, (c) after an exposure of 0.4 L of de-acetone at 85 K, (d) after an exposure of 0.4 L acetone to a surface precovered with hydrogen (0.25 monolayer). (B) Thermal desorption spectra (1 K/s) following acetone adsorption (1.6 L exposure) on Pt(ll1): (a) hydrogen (mass 2) (b) CO (mass 28) (c) acetone (mass 43).
MA. Vannice et al. / A RAIRS and HREELS study of acetone on Pt(lllJ
comparing a few infrared spectra to those obtained by backfilling the chamber with acetone. All exposures were at 85 K unless otherwise noted. Infrared spectra with 4 cm-’ resolution were taken by averaging 660 scans over 4 min. As the MCT infrared detector has a cut-off frequency of 800 cm-‘, HREELS (AE = 40 cm-“) was used in addition, mainly to get spectroscopic information in the low frequency range. For the TD spectra a heating rate of either 3 or 1 K/s was used, as stated in the figures. All RAIRS and HREELS data were obtained at 85 K unless otherwise noted.
f
*
Acetone
on
3 Pt (III)
3. Results Thermal desorption (TD) spectra were obtained for acetone on the clean Pt(ll1) surface as well as on one preexposed to 3 L H, at 90 K. The latter surface represents a hydrogen coverage of about 0.25 according to the work of Christmann and Ertl [S]. For acetone the major fragmentation peak at 43 amu (46 amu for acetone-d,) was monitored. Fig. 1A shows the TD spectra for multilayer and monolayer coverages of acetone, for near-monolayer coverage of completely deuterated acetone, and for acetone coadsorbed with hydrogen using a heating rate of 3 K/s. The multilayer phase desorbs near 149 K while the monolayer state desorbs near 199 K. The presence of hydrogen on the surface lowers these peaks by about 9 K to 140 and 189 K, respectively. The principal monolayer peak at 185 K for d, acetone is lower by about 13 K. To improve the resolution of more than 1 desorption peak near 200 K, a heating rate of 1 K/s was also used. In addition, to determine if any H, or CO was desorbing, masses at 2 and 28 amu were monitored as well as mass 43, and these spectra are shown in fig. 1B. At this heating rate well-resolved peaks at 130, 184 and 199 K were obtained, and little or no decomposition to CO and hydrogen was observed. The TD spectra are raw, unsmoothed data obtained with a feedback loop which provided a linear heating rate, and some perturbations were sometimes observed during the initial startup period because of this feedback loop. These baseline fluctuations in the lowest temperature region could
1 %,
r
I,
I
,“,I,
1,
zoo0
I
looo WAVENUMBER
b-i’)
Fig. 2. RAIR spectra at 85 K of acetone adsorbed on Pt(ll1): (a) after an exposure of 1 L (7’ i-multilayer species), (b) after an exposure of 2 L, (c) after an exposure of 3 L, (d) after an exposure of 4 L (multilayer species), (e) after a flash to 120 K of the preceding surface, (f) after a second flash to 140 K of the preceding surface.
produce shoulders at times, such as in spectrum a, fig. 1A. Infrared spectra were obtained after increasing acetone exposures at 85 K. For acetone on a clean Pt(l11) surface following the final exposure of 4 L, the crystal was flashed to 120 K (10 K/s) and an IR spectrum was obtained, then it was flashed to 140 K and a final spectrum was run. This series of IR results is shown in fig. 2. The peaks in fig. 2 spectra b, c and d are near those for liquid-phase acetone [6,7] and easily associated with multilayer acetone [l]. To better resolve the vi monolayer phase, which is associated with the C=O stretching frequency at 1638 cm-’ [l], a lower exposure of 0.4 L was used. The sample was again flashed to 140 K after this measurement and these two spectra are provided in fig. 3. The high sensitivities required to resolve IR peaks for submonolayer coverages of acetone should be noted; for exam-
4
M.A. Vannice et al. / A RAIRS
and HREELS
ple, the 2060 cm-’ peak for CO in fig. 3, spectrum b, represents a coverage of approximately 0.001 monolayer of CO. To examine the possibility that the $ species can convert to the n* species at temperatures near the desorption temperature, the crystal was held at 170 K to prevent multilayer formation but retain the monolayer species, acetone was adsorbed, and a spectrum was obtained at this temperature. The crystal was warmed to 180 K and then to 200 K with an IR spectrum obtained at each temperature. These three spectra are shown in fig. 4. As anticipated, the n’ phase was the principal one present at the two lower temperatures, but the peak at 1646 cm-’ for the C=O stretch had disappeared at 200 K although new bands in the carbonyl region at 1612,1584, and 1511 cm-’ had grown in and bands associated with the methyl groups still remained. Spectra for deuterated acetone, (CD,),CO, were obtained under conditions identical to those used
I
Acetone
on Pt (111)
WAVENUMEIER hi’ 1 Fig. 3. RAIR spectra at 85 K of acetone adsorbed on Pt(ll1): (a) after an exposure of 0.4 L ( TJ’ species), (b) after flashing the preceding surface to 140 K (7’ species).
study of acetone on Pt(1 I I)
Acetone
2oal
on Pt (111)
loo0
WAVENUMBER km-‘) Fig. 4. RAIR spectra of acetone adsorption on Pt(ll1) at higher temperature: (a) at 170 K following an exposure of 0.4 L at 170 K (primarily 9’ species), (b) at 180 K after warming the preceding surface, (c) at 200 K after further warming of the preceding surface ($ species).
for (CH,),CO. The increase in band intensities with increasing exposure at 85 K on Pt(ll1) is shown in fig. 5 along with the state of the surface after a flash to 170 K following the last cumulative exposure of 0.32 L. After a 0.4 L exposure only the monolayer state was present, but as previously observed, as the multilayer peak (1710 cm-‘) grew, the 1626 cm-’ peak associated with the n’ species decreased significantly. The sensitivity and resolution capability of the RAIRS technique is particularly apparent in this series of runs as the various deformation and rocking modes of the -CD, groups are nicely resolved in the 9501050 cm-’ region. Because of the interest in the reaction between acetone and H, to form isopropanol [2], an approximate hydrogen coverage of 0.25 monolayer was established and acetone was adsorbed on this surface. An exposure of 0.4 L gave the monolayer state with only a small peak at 1712 cm- I, which
M.A. Vannice et al. / A RA IRS and HREELS
study of acetone on Pt(llIf
is
representative of the C=O stretch frequency in a multilayer phase. A further cumulative exposure to 0.8 L increased the intensity of the 1712 cm-’ peak and reduced the intensity of the 1640 cm-’ peak, as shown in fig. 6. This concomitant decrease in the 1640 cm-’ peak as the 1712 cm-’ peak increased thus reproduced the trend in figs. 2 and 5 with acetone alone on a clean Pt surface. HREEL spectra were taken after exposures identical to those used for the IR results. Spectra a and d in fig. 7 show the EELS bands present after 0.4 and 3 L exposures, respectively. After each of these spectra was taken, the surface was flashed to 170 K and another spectrum was obtained, and these are presented in fig. 7, spectra b and e. Finally, the EEL spectrum shown in fig. 7, spectrum c, was taken for the acetone coadsorbed with hydrogen immediately after the IR spectrum in fig. 4, spectrum a, was obtained. EEL spectra were
Acetone
5
on t-i-covered
Pt (111)
a
b
1”
s r
I’
‘I1
2m WAVENUMBER Acetone
on Pt (111)
b
I” ?axl
(cm-‘)
Fig. 6. RAIR spectra at 85 K of acetone coadsorbed with hydrogen on Pt(ll1): (a) after exposure of a surface precovered with 0.25 monolayer hydrogen to 0.4 L acetone, (b) after exposure of the preceding surface to an additional 0.4 L acetone.
also acquired for deuterated acetone after exposures of 0.4 and 0.8 L and then after a flash to 170 K following the final exposure. These spectra are given in fig. 8. In these two figures, the specular peak heights represent count rates between 4 x lo4 and 2.3 X lo5 counts per second, as noted in the figure captions.
I
‘2doo’
.
*
’
I
WAVENU~BER Fig. 5. RAIR spectra at 85 after an exposure of 0.4 L (q’ 0.8 L, (c) after an exposure preceding surface
c
I
t
d
:3
v*.
kx-i’J
K of d,-acetone on Pt(ll1): (a) species), (b) after an exposure of of 1.6 L, (d) after flashing the to 170 K (q2 species).
4. Discussion The only study of acetone adsorbed on Pt reported in the literature is that of Avery, who employed TDS and EELS to identify different adsorbed states on a Pt(ll1) surface [l]. Two species were proposed to exist up to monolayer coverage - an $ species bonded end-on through the oxygen atom and an q* species adsorbed with the C=O bond horizontal to the surface in either a a-bonded or d&-bonded configuration. Similar
6
MA.
Vannice et al. / A RAIRS
and HREELS
study
species have subsequently been proposed to exist on the Ru(001) and Pd(ll1) surfaces [8,9]. Whereas multilayer acetone on Pt(ll1) desorbed at 137 K, the 7’ species desorbed at 185 K and the n2 species was proposed to desorb over a range of
Acetone
of acetone on Pt(lI1)
.,ooo Acetone-d,
on Pt (111)
on Pt (111)
JL
I
0
m WAVENUMBER
2030 [cm-‘)
Fig. 8. HREEL spectra at 85 K of de-acetone [(CD,),CO]: (a) after an exposure of 0.4 L (TJ’ species) (4.2 x lo4 cps), (b) after an exposure of 0.8 L (9.8~10~ cps), (c) after flashing the preceding surface to 170 K (4.1 x lo4 cps).
t:
wjkky 3cKm WAVENUMBER
!I
km-‘)
Fig. 7. HREEL spectra at 85 K of acetone adsorbed on Pt(ll1); (the number in parentheses for each spectrum represents the specular peak height in counts per second): (a) after an exposure of 0.4 L (r$ species) (1.7 ~10~ cps), (b) after flashing the preceding surface to 170 K (9.5 X lo4 cps), (c) after exposure of a surface precovered with 0.25 monolayer hydrogen to 0.4 L acetone (4.3 x lo4 cps), (d) after an exposure of 3 L (multilayer species) (2.2~ lo5 cps), (e) after flashing the preceding surface to 170 K (2.3 X lo5 cps).
temperatures above 200 K based on rather unusual TD spectra [l]. In the TD spectra in fig. 1, the peaks for the multilayer and monolayer phases are clearly resolved by well-behaved TD curves, with the peak temperatures increased by 16-18 K at the higher heating rate. At the heating rate used by Avery (1 K/s), our desorption peak at 184 K for the n’ species nicely reproduces his value of 185 K, and the multilayer peak at 130 K is near his value of 137 K. These peaks are relatively sharp, and at the lower heating rate an additional peak at 200 K can be resolved. Unlike previous TD spectra for acetone on Pt(ll1) [l], Ru(001) [8], and Pd(lll) [9], the spectra in fig. 1 exhibit no tailing and thus indicate the absence of higher-energy binding
1370 1235 1105 910
1373 1239 1094
900
1360 1220 1090 1065 900 783 530 484 390
WW
a) Me designates a methyl group. b, From refs. [5,6]. ‘) From ref. (131.
WO) r(CO) G(MeCMe)
v,(MeCMe)
P We)
P(Me)
v,(MeCMe)
405
550
1430
1443 1422
1445 1428
WW
1350 1238 1086
1426
810 570
1355 1250 1080
1440
1640
1638
2920 1715
1716
3005
3010
3005 2960 2925 1712
v(W
$-acetone
1365 1240 1086
1610 1530-1585 1511 1420-1445
IR
EELS
From IPA ‘)
830
1550
1360-1410 1262 1085
1611 1584 1512 1460 1004
IR
1085 1245 960 887 724 690 480 405 326
1035
2255 2222 2120 1700
Liquid IR b,
IR EELS
q’-acetone
EELS
IR
Liquid IR b,
Multilayer
(CD,),CO
(CH,)aCO
for acetone on Pt(ll1)
We)
Mode ‘)
Vibrational mode assignments (cm-‘)
Table 1
1090 1261 960
1036
1710
IR
1690
EELS
Multilayer
1280 958
1040 1025 992 983
1626
IR
850 715 520
1275
985
1030
1620
2230
EELS
+-acetone
IR
1262 959
959
1039
1611 1512
$-acetone
a’: 3 3. w
% ?-
8
M.A. Vannice et al. / A RA IRS and HREELS
states (extending up to 250 K on Pt and 400 K on Ru and Pd) that have been as>ociated with the q2 species on these surfaces. The absence of any binding states above 220 K could be a consequence of a cleaner surface or, more likely, a much lower concentration of defect sites, as the n2 species has been proposed to adsorb on step sites on the Pt(ll1) surface [l]. The results in fig. 1 also imply that if the 9’ state can convert to the n2 species, it does so in the same temperature range that the desorption process occurs thus making it much more difficult to isolate the n2 species on Pt(ll1) compared to the Ru(001) and Pd(ll1) surfaces. Using the Redhead equation with v = 10” s-’ [lo], desorption energies of 12.5, 11.5, and 8.0 kcal/mol can be calculated from fig. 1B for the n2, n’, and multilayer acetone species, respectively. The last value is very close to the heat of vaporization of 7.64 kcal/mol for acetone [ll]. This small difference in binding energy between the 7’ and n2 states is one reason why it is difficult to form and isolate the latter species on Pt(ll1). Little or no decomposition of acetone occurred on this surface, as indicated by the absence of a CO peak near 460 K [12] and any significant H, peak, and this may again be due to the absence of defect sites. In other TDS runs showing the desorption of acetone and H, formed by the decomposition of isopropanol on this surface, no CH, was observed [13]; consequently, we do not expect any CH, formation here. The IR spectra for both the monolayer and multilayer states give peak positions in excellent agreement with the assignments made by Avery [l]. As illustrated in table 1, the IR peaks for the multilayer state agree closely with the bands reported for liquid-phase acetone [6,7]. The multilayer HREELS peaks are also included and, although their resolution is lower, they are in excellent agreement with the IR peaks and correspond well to the liquid-phase positions. There is again a one-to-one correspondence between the IR and EELS peaks that are associated with the 17’ state, and they also agree extremely well with those reported by Avery. The C=O stretch frequency exhibit a red shift of about 75 cm-‘, while smaller blue shifts (15-30 cm-‘) occur for the C-C-C
study of acetone on Pt(l II)
skeletal modes, and the methyl groups are essentially not affected. The same trends exist for the deuterated 7’ acetone species, whose peak positions are also listed in table 1. All these assignments have been discussed in detail [1,8,9]. Our principal intent in this study was to examine the IR spectra as well as the HREEL spectra for evidence substantiating the presence of an n2 species. An intriguing pattern was always observed with increasing exposure to acetone. A comparison of spectrum 3a with those in fig. 2 shows that the development of the multilayer state results in the disappearance of the n* state because the 1638 and 1350 cm-’ bands associated with the latter disappear and are replaced by the 1716 and 1373 cm-’ multilayer peaks. High exposures are not required for this transformation; for example, as shown in fig. 5, spectrum c, an exposure of less than 2 L noticeably suppresses the CO band in the n’ (CD,),CO species (1626 cm-‘) while the same band for the multilayer species at 1710 cm-’ grows significantly. Two explanations can account for this curious behavior: either the 9’ species interacts more strongly with the polar acetone molecules in the second layer than with the Pt surface, thus removing the end-on bonding alignment with the Pt surface, or a restructuring of the 71’ phase occurs to create a monolayer phase with the C=O bond parallel to the surface thus reducing the normal component of the dipole moment and making it more difficult to detect. The latter possibility would produce an n* species, and it may be more likely because the desorption energies indicate a stronger interaction between acetone and the Pt surface than between acetone molecules. The IR spectra for the monolayer state of acetone on Pt(ll1) repeatedly exhibited peaks between 1500 and 1610 cm-’ that were difficult to resolve with HREELS because of their low intensity. One example is provided by a comparison of fig. 3, spectrum a, in which bands at 1609 and 1520 cm-’ can be seen, with fig. 7, spectrum a, in which these two bands are not resolved. It is possible that these weak peaks could represent n2 species in different configurations [1,X]. To determine if conversion from the n’ to the n2 species could occur at higher temperatures, as reported
M.A. Vannice et al. / A RA IRS and HREELS
for the Pd(ll1) surface [9], the monolayer coverage in fig. 3, spectrum a, was flashed to 140 K and re-examined. Spectrum 3b shows that the 1638 yet bands between cm ’ band had disappeared, 1550 and 1610 cm-’ remained while the 1429 and 1086 cm-’ peaks associated with deformation and rocking modes of the methyl groups had grown substantially. The retention of position for the methyl peaks at 1086, 1360 and 1428 cm-’ establishes that an intact acetone species remains, but it has no detectable peak for the C=O stretch at 1638 cm-‘. In addition, the intensities of these three bands increase rather significantly as the 1640 cm-’ peak vanishes. This same trend can be seen in the HREELS spectra in fig. 7, spectra a and b. Although not all the 7’ species was removed, the 1640 cm-’ peak diminished noticeably while the bands at 830, 1085, and 1420 cm-’ increased in intensity. Also, a 1550 cm-’ peak was resolved in spectrum 7b. The intensities of the peaks associated with the methyl groups after flashing to 140 or 170 K indicate that rather high surface coverages remain and, in fact, the appearance of the 1711 cm-’ peak in fig. 3, spectrum b, for multilayer acetone implies that the surface must have nearly saturated with species constituting monolayer coverage. During the rapid flash some CO formation occurred to give the bands at 2050 and 1806 cm-‘, which are associated with CO adsorbed on on-top and 3-fold hollow sites, respectively [14]. This decomposition is attributed to the hot tungsten wire wrapped around the edge of the Pt crystal and used for heating rather than to the Pt surface itself. For example, no CO was observed in fig. 1B when a much lower heating rate was used. The band near 1610 cm-’ and those between 1520 and 1680 cm ’ cannot be associated with isopropanol [13] and are almost certainly associated with carbonyl bonds, thus they may possibly represent more strongly coordinated n2 species. Further support for this assignment comes from the IR spectra in fig. 4, which represent acetone adsorbed at 170 K, rather than at 85 K, and subsequently warmed gently to 200 K. The initial state at 170 K gives a spectrum quite similar to that of fig. 3, spectrum a, with bands near 1610 and 1500 cm-’ being distinguishable along with those expected
study of acetone on Pt(l I I)
9
for the $ state. Warming to 200 K removed the 1646 cm-’ peak but increased the band intensities at 1610, 1584, and 1511 cm-‘, while bands near 1425 and 1360 cm-‘, associated with the methyl groups, remained. The growth of the 2070 cm-’ peak again indicates that some decomposition to CO was occurring but again this is attributed to the hot tungsten wire. The peak at 2080 cm-’ in fig. 4, spectrum c, represents a surface coverage of about 0.1 monolayer CO. The possibility that the three peaks between 1500 and 1610 cm-’ represent CO adsorbed on high-coordination sites cannot be discounted; for example, very weak peaks at 1410 and 1650 cm-’ have been reported for CO on a Pt 6(111) X (111) surface at low coverage [12]. However, at this time we do not think these peaks represent molecular CO species for the following reasons. First, these low-coordination sites were not observed in a HREELS study of CO on Pt(ll1) [12]. Second, RAIR spectra of CO adsorbed on our Pt(ll1) surface showed no peaks in this region. Third, the intensities of the bands between 1500 and 1610 cm-’ do not correlate with the intensity of the CO band near 2070 cm-‘, as shown by a comparison of figs. 2 (spectrum a), 3 (a and b) and 6 (b). Finally, the decomposition of isopropanol on this crystal surface also produced peaks at 1611, 1584, and 1512 cm-’ without the formation of CO [13]. Consequently, these bands may be representative of n2 species. The IR spectra in fig. 5 for d,-acetone showed well-resolved bands in the 950-1050 cm-’ region that are associated with the deformation and rocking modes of the -CD, groups. Again the growth of the multilayer peak at 1710 cm-’ suppressed the 1625 cm-’ peak of the monolayer species, and the C-C-C skeletal mode at 1261 cm-’ of the multilayer deuterated species became especially intense. Flashing to 170 K left well-defined peaks at 1039 and 959 cm-’ that were stronger than those at monolayer coverage (fig. 5, spectrum a) along with a band at 1262 cm-’ indicating that intact acetone species were still present. Peaks at 1611 and 1512 cm-’ were again observed. The peaks in the 1360-1430 cm-i region are attributed to a small amount of nondeuterated acetone displaced from the walls of the chamber by d,acetone and readsorbed on the Pt surface.
10
MA. Vannice et al. / A RAIRS and HREELS study of acetone on Pt(l li)
To examine the influence of coadsorbed hydrogen, acetone was adsorbed on a surface precovered with 0.25 monolayer hydrogen. The spectrum after a 0.4 L exposure (fig. 6, spectrum a) is similar to that of acetone on the clean surface after the same exposure; however, the adsorption capacity for the 7’ species was somewhat reduced because the 1640 cm-’ band on the H-covered surface was less intense as the multilayer 1712 cm-” band began to form. An exposure of 0.8 L gave both mono- and multilayer species, as shown in fig. 6, spectrum b, but a distinct peak near 1530 cm -I can also be observed. A similar set of experiments was conducted with HREELS on two surfaces: one precovered with 0.25 monolayer of hydrogen and the other precovered with 0.5 monolayer. The HREEL spectrum for acetone on the former H-covered surface is shown in fig. 7, spectrum c, and it has particuiarly good peak resolution. Although there are some small shifts in the peak positions, there is a one-to-one correspondence between the IR and HREEL peaks above 1000 cm - ’ and the rf species can easily be identified. The HRBEL signal was much weaker for acetone adsorbed on the surface coveted with 0.5 monolayer H, thus indicating that the surface coverage was substantially lower and that hydrogen and acetone compete for the same sites, as indicated by kinetic models for acetone hydrogenation on Pt [2]. Regardless, bands at 1620, 1410, 1070, 830, and 560 cm-’ could be clearly resolved. After flashing this latter surface to 170 K, the 1620 cm-’ peak diminished, the 1410, 820, and 560 cm-’ peaks were relatively unchanged, the 1080 and 910 peaks associated with methyl group rocking modes and the 410 cm-’ peak associated with a skeletal deformation mode intensified, and a weak band near 1550 cm-’ could be identified. The peaks that have been observed for adsorbed monolayer acetone in the absence of the $ species (i.e., no 1640 cm-’ peak), and which could therefore represent bands associated with nz species, are summarized in table 1, They have been obtained primarily from figs. 3 (spectrum b), 4 (c), 5 (d) and 6 (b). The principal difference between these peaks and those found for the T.# species are the carbonyl bands near 1511, 1550,
and 1610 cm-‘. These are tentatively assigned to adsorbed acetone species more strongly coordinated to the surface than the 9’ species, and whose carbonyl stretch mode is much more difficult to detect, thus these bands may represent the n2 species proposed by Avery [l]. These carbonyl bands do have higher frequencies than those assigned to n2 species on Ru(001) by Anton et al. [8] and on Pd(ll1) by Davis and Barteau [9]; however, Lercher et al. have associated bands at 1580 and 1520 cm-l with a “side-on” configuration for acetone adsorbed on MgO [16]. In the study of Anton et al., the 1275 cm-’ peak for de-acetone, which was associated with the C=O stretch frequency, corresponds to the C-.C.-C skeletal stretch frequency of 1280 cm-’ on Pt(l11) in fig. 5 and it is near the bands of 1260-1300 cm-r on Pd(ll1) [9]. Since the carbonyl band stretch intensity of any species with the C=O bond parallel to the surface is expected to be weak [15], it is possible that the relative growth of this band on Ru as the carbonyl band at 1665 cm-’ disappears is a consequence of an enhanced contribution from the skeletal mode. Regardless, it is still indicative of an n2 species. The CO stretch frequency near 1440 cm-’ assigned to an n2 species on Pd(ll1) [9] is not too much lower than the bands observed on our Pt(ll1) crystal #I. In conclusion, there is evidence, although it is not easily obtained, that an adsorbed acetone species exists on a Pt(ll1) surface with its C=O bond oriented such that it is difficult to detect with vibrational spectroscopy. Thus the conclusion by Avery {l] that such an n2 species occurs on Pt is supported by this study. The evidence consists of: (1) a TD peak at 199 K attributed to an n2 species in addition to that at 184 K for the n’ state, (2) the disappearance of the 1640 cm- ’ peak for the T# species as the multilayer phase forms, (3) the retention and enhancement in methyl peak intensities in the absence of the 1640 cm -I band,
One reviewer raised the possibility that bands between 1500 and 1600 cm’-’ may represent acetyl (CH,C=O) species; however, McCabe et at. have associated bands at 610, 900-925, 1120 and 1650 cm-’ with this species on a Pt(S~-[6(lll)X(l~)~ surface 1171, thus we do not believe this species is present.
M.A. Vannice et al. / A RA IRS and HREELS
(4) the appearance of weaker bands between 1500 and 1610 cm-’ as the 1640 cm- ’ band decreases or disappears, and (5) the generation of these same lower-frequency bands during the decomposition of isopropyl alcohol to hydrogen and acetone [13].
5. Summary The use of RAIRS has provided the first IR spectra of acetone adsorbed on an unsupported metal surface. These results were complemented by HREELS to cover the frequency range between 200 and 3500 cm-‘. The RAIR spectra readily resolved all the peaks expected from liquid-phase IR spectra and gave very similar peak positions for all the multilayer acetone bands whereas the 17’ monolayer state had a C=O stretch frequency red-shifted to 1638 cm-‘. Direct evidence for an nz species, proposed to exist with its C=O bond parallel to the surface, was not easy to obtain with our essentially defect-free Pt(ll1) surface because of the small (1 kcal/mol) difference in binding energies between the TJ~ and q2 states and the absence of higher-energy binding states for the n2 species that have been reported for other metal surfaces. However, a second desorption peak at 199 K could be resolved from the T/ peak at 184 K, and this more strongly bound acetone species is associated with an n2 state. TDS also showed that no acetone decomposition occurred during desorption. Additional evidence for a monolayer species other than the 7’ phase was provided by the RAIRS and HREELS results. First, the n1 species that initially forms gradually disappears as the multilayer phase develops. Second, after flashing to temperatures below 200 K the 1638 cm-’ band is absent yet peaks at 1086, 1360 and 1428 cm-‘, associated with the methyl groups, not only are retained but also grow in intensity. Furthermore, new bands between 1500 and 1610 cm-’ appear.
study of acetone on Pt(l I I)
11
Finally, these latter bands have also been observed after the dissociative adsorption of isopropyl alcohol to give hydrogen and acetone [13].
Acknowledgements One of us (M.A.V.) would like to thank not only the Alexander von Humboldt-Stiftung for a Senior Scientist Research Award during the period this work was conducted, but also the IGV Forschungszentrum Jtilich for making his stay there possible. The assistance of C. Damarow in the laboratory was also greatly appreciated.
References 111N.R. Avery, Surf. Sci. 125 (1983) 771. 121B. Sen and M.A. Vanmce, J. Catal. 113 (1988) 52. [31 W. Erley, J. Electron. Spectrosc. Relat. Phenom. 44 (1988) 65. [41 U. Linke and B. Poelsema, J. Phys. E (Sci. Instrum.) 18 (1985) 26. [51 K. Christmann and G. Ertl, Surf. Sci. 60 (1976) 365. 161 T. Shimanouchi, Tables of Molecular Vibrational Fre39 (US quencies, Consolidated Vol. I, NSRDS-NBS Government Printing Office, Washington, 1972). Atlas of 171 B. Schrader and W. Meier, Eds., Raman/IR Organic Compounds, Vol. 1 (Verlag Chemie, Weinheim, 1977). PI A.B. Anton, N.R. Avery, B.H. Toby and W.H. Weinberg, J. Am. Chem. Sot. 108 (1986) 684. 191 J.L. Davis and M.A. Barteau, Surf. Sci. 208 (1989) 383. WI P.A. Redhead, Vacuum 12 (1962) 203. [111 R.C. Weast, Ed., CRC Handbook of Chemistry and Physics, 59th ed. (CRC Press, Boca Raton, 1978). WI H. Hopster and H. Ibach, Surf. Sci. 77 (1978) 109. 1131 M.A. Vanmce, W. Erley and H. Ibach, Surf. Sci. 254 (1991) 12. 1141 B.E. Hayden and A.M. Bradshaw, Surf. Sci. 125 (1983) 787. P51 H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982). iI61 J.A. Lercher, H. Noller and G. Ritter, J. Chem. Sot. Faraday Trans. I, 77 (1981) 621. P71 R.W. McCabe, CL. DiMaggio and R.J. Madix, J. Phys. Chem. 89 (1985) 854.