Langmuir 1987,3,555-557 Scheme I1 D3C\r /CD3
I
on Ti02.12 Three lines of evidence implicate the 1585-cm-I band as acetone enolate: (i) comparison with model compounds (Table I) which places the C=C stretch in the 1560-1630-~m-~ region, (ii) the concurrent exchange of surface OH with deuterium of a ~ e t 0 n e - dwith ~ the development of the band at 1585 cm-', and (iii) the fact that acetone enolate is a rational precursor to mesityl oxide and isophorone which appear more slowly in the infrared spectrum (Scheme I). The presence of mesityl oxide and isophorone is identified by independent adsorption of these molecules and it is suggested that formation of isophorone is responsible for the more rapid growth of the intensity in the CH vs. the C=O, C=C region of the infrared spectrum. The enolization of acetone on y-alumina as represented in Scheme I requires the close proximity of both Lewis acid and Lewis sites on the surface. Models for the surface of
555
partially dehydroxylated y-alumina indicate that this requirement is ~ a t i s f i e d . Mesityl ~ ~ ~ ~ ~oxide is formed via nucleophilic attack of the enolate on an adjacent adsorbed acetone molecule followed by dehydration. Attack of acetone enolate on adsorbed mesityl oxide gives a linear trimer of acetone which may then cyclize to give isophorone. Addition without cyclization will yield oligiomers of acetone which may not be thermally desorbed during TPD (vide supra-Figures 4 and 5). All of the surface species shown in Scheme I are capable of polymerization under a variety of conditions. This is particularly true for mesityl oxide. Methyl iodide is observed to displace many of the above species as described in the discussion of Figure 6. This may be explained by formation of AI-0-CH, and A1-I bonds on the surface. Scheme I1 represents this reaction for the displacement of acetone but may be generalized for the displacement of other adsorbed ketones.
Acknowledgment. Support of this work was provided by NSF through an industrial-university cooperative grant (DMR 85 18364). (24) Knozinger, H.; Ratnosamy, P. Catal. Rev. Sci. Eng. 1978,17, 31. (25) Peri, J. B. J. Phys. Chem. 1965, 69, 211, 220.
Thiophene Chemisorption and Thermal Decomposition on Nickel ( 100) Single- Crystal Surfaces F. Zaera* Brookhaven National Laboratory, NSLS Department, Upton, New York 11973
E. B. Kollin and J. L. Gland Exxon Research and Engineering Co., Corporate Research Science Laboratory, Annandale, New Jersey 08801 Received July 12, 1986. In Final Form: February 16, 1987 Thiophene chemisorption and decomposition over Ni(100) have been studied by using HREELS and TDS. The C-S bond breaks below 90 K, and a metallocycle forms on the surface. We propose that the metallocycle has C4H3stoichiometry and is adsorbed at an angle to the metal surface. This moiety is stable up to 500 K, when further dehydrogenation takes place. Understanding the chemisorption of sulfur-containing molecules on metal surfaces is an important step in developing our understanding of catalytic C S bond cleavage. However, few studies have been performed to date using the techniques of modern surface science. Thiophene chemisorption and decomposition over Mo(100)l and Pt( 1 1 1 ) 2 9 3 have been studied by using a variety of techniques such as TDS, HREELS, and NEXAFS. Brief reports on the adsorption of thiophene on other metal crystals can also be found in the literature.&' In the present paper, we report thermal desorption and vibrational results on the chemisorption and decomposition of thiophene on Ni(100). The experiments were performed in a stainless steel ultrahigh vacuum chamber described in detail elsewhere.s The apparatus is equipped with 127' deflection *Permanent address: Department of Chemistry, University of California, Riverside, CA 92521.
0743-7463/87/2403-0555$01.50/0
cylindrical electron energy monochromator and analyzer for HREELS, a shielded quadrupole mass spectrometer with a coaxial entrance tube for TDS, and a hemispherical analyzer for Auger and X-ray photoelectron spectroscopies. The Ni crystal was cut and polished along the (100) plane by standard procedures, and mounted on a manipulator (1) Zaera, F.; Kollin, E. B.; Gland, J. L. Surf. Sci., in press. (2) Stohr, J.; Gland, J. L.; Kollin, E. B.; Koestner, R. J.; Johnson, A. L.; Muetterties, E. L.; Sette, F. Phys. Reu. Lett. 1984, 53, 2161. (3) Koestner, R. J.; Kollin, E. B.; Gland, J. L., submitted for publication in Surf. Sci. (4) Edmonds, T.; McCarroll, J. J.; Pitkethly, R. C. Ned. Tijdschr. Vacuumtech. 1970,8, 162. (5) Gellman, A. J.; Farias, M. H.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1984, 136, 217. (6) Richardson, N. V.; Campuzeno, J. C. Vacuum 1981, 31, 449. (7) Wexler, R. M.; Gentle, T. M.; Muetterties, E. L.; private communications. (8) Zaera, F.; Hall, R. B. Surf. Sci. 1987, 180, 1.
0 1987 American Chemical Society
556 Langmuir, Vol. 3, No. 4, 1987
Zaera et al. ad-CaDaH&Nl(lOO)
HREELS TfKI
TDS
e ,,
$1
1
2000
Frequencykm
3000 1
50
250
450
650
850
50
TemperatufeIK
Frequency/cm
250
1
450 650 Temperatu re/K
850
Figure 1. HREELS and TDS from saturation C4H4Son Ni(100). HREEL spectra were taken at 90 K after annealing to the indicated temperatures.
Figure 2. HREELS and TDS from saturation C4DzHzS-a-dz on Ni(100). HREEL spectra were taken at 90 K after annealing to the indicated temperatures.
Table I. Vibrational Assignment for Thiophene Chemisorbed on Ni(100) at 90 K (X Stands for H or D) mode CaHdS CADdS CdDzHzS-a-dZ VNiS 355 290 YR, BZ 465 420 545 595 xcx, B2 735 755 835 745 PCX in, AI (a) TCXX' out, Bz 840 825 Ycx in, A1 1080 1035 1040 1250 1210 ~ C X in, B, 1260 P'CX in, A, (P) 1250 1305 "CC, A, 1400 1380 1380 "'CCI B, 1585 1435 1480 "CX 3115 2320 2320, 3085
Table 11. Vibrational Assignment for Thiophene Chemisorption on Ni(100),T = 230-500 K
so it could be cooled below 90 K and resistively heated above 1500 K. This sample was cleaned by argon ion sputtering in order to remove sulfur contamination and by cycles of oxygen treatment at 1100 K and annealing to 1300 K until no impurities were detected by HREELS, AES, or XPS. Thiophene (Aldrich, gold label, 99+ %) and thiophene-2,5-d2 and -dq (MSD, 97+ % isotopic purity) were used as supplied after outgassing by repeated freeze-pumping cycles. Exposures were done by using dosers terminated with capillary arrays which created high local pressures at the front face of the sample. All experiments reported here were done after thiophene saturation (about 5 langmuirs). Thermal desorption and vibrational spectra obtained after thiophene saturation of the Ni(100) surface are shown in Figure 1. The 84 amu TDS (molecular thiophene) displays two peaks, one at 145 K due to multilayer desorption, and a second peak at 175 K. Hz thermal desorption due to thiophene decomposition shows a multitude of peaks, starting with one at 235 K and followed by a series of overlapping peaks between 300 and 650 K. HREEL spectra obtained after annealing at selected temperatures are also shown in Figure 1. Although the Hz TDS looks complicated, the vibrational spectra obtained after annealing to temperatures in the 230-500 K range look very similar, suggesting the existence of a stable and unique surface intermediate in that temperature range. In order to facilitate the vibrational assignment of the HREEL spectra, experiments were also performed with a-deuteriated and fully deuteriated thiophene. TDS and HREELS obtained by using thiophene-a-d2 are shown in Figure 2. The thermal desorption spectra indicate that the first peak at 240 K is due to a-D abstraction, since it only appears in the Dz trace. The vibrational frequencies
mode "NiS
xcx
6cx in (central) 6,, in (terminal)
ycc "'CC
"CX
CdHLS 360 850 1070 1340 1150 1450 3025
CdDdS 360 645 810 1100 1425 2260
C,D7H,S-a-d7 345 820 1050 1280 1420 2200, 2995
seen for all three compounds and their assignments are summarized in Tables I and 11. HREELS after chemisorption at 90 K display the vibrational modes expected from molecular thiophene? although the relative intensities of the peaks are different from those obtained from a thick multilayer.' This suggests that adsorption near the surface may have a preferential orientation. The high intensity of the peak at 735 cm-' compared to that for the 835-cm-' bond for normal thiophene indicates that the molecular ring may be nearly perpendicular to the surface. All vibrational spectra at 90 K present an extra feature around 300 cm-' which corresponds to a metal-sulfur stretch. Near-edge (NEXAFS) and extended X-ray absorption (SEXAFS) experiments have established unequivocally that the first layer of thiophene decomposes on Ni(100) even at 90 K, leaving atomic sulfur and a carbonaceous fragment on the surface.1° This is consistent with the M-S stretching frequency observed here and could also explain the appearance of extra features in the HREELS above 1200 cm-'. After the sample is heated above 250 K, molecular thiophene desorbs and a-dehydrogenation of the remaining fragment takes place. The area under the 240 K peak in the 4 amu TDS for thiophene-a-dz, and the relative intensities for the C-D (2200 cm-') and C-H (2995 cm-') peaks in the HREELS (1:2), indicates that only one of the deuterium atoms has been removed from the ring at this point. Assignment of the vibrational modes was done with the help of data from few reference compounds, listed in Table 111. Our proposed structure for the intermediate (9) Scott, D. W. J . Mol. Spectrosc. 1969, 31,451. (10) Stohr, J.; Kollin, E. B.; Fischer, D. A.; Hastings, J. B.; Zaera, F.; Sette, F. Phys. Rev. Lett. 1985, 55, 1468. (11)Harris, R. K. Spectrochirn. Acta 1964,20, 1129. (12)Davidson, G.Znorg. Chim. Acta 1969, 3, 596. (13)Benedetti, E.; Aglietto, M.; Vergamini, P.; Aroca Munoz, R.; Rodin, A. V.; Panchenko, Yu. N.; Pentin, Yu. A. J.Mol. Struct. 1976,34,
Thiophene Chemisorption and Thermal Decomposition
mode
Langmuir, Vol. 3, No. 4, 1987 557
Table 111. Vibrational Assignment for Reference Compounds CIHsFe(CO) i2 ~,~-1,4-C4HdCl;~
1,3-butadiene1* (trans)
(cis, C d
C4H4S/Ni(100) 230-500 K
908 (A,) 911 (B,)
896 (A”) 926 (A’)
central
967 (B,) 1013 (A,)
669 (A’) 791 (A“)
6 c in, ~ central
1291 (A,) 1296 (B,)
1060 (A’) 1174 (A”)
1185 (B,) 1232 (A,)
1070
YCHz Or 6CH terminal
1385 (B,) 1442 (A,)
1370 (A”) 1499 (A‘)
1305 (B,) 1425 (A,)
1340
sym a5Ym
1205 (A,) 1599 (B,) 1643 (A,)
1205 (A’) 1439 (A”) 1477 (A’)
1065 (A,) 1574 (B,)
1150 1450
yCH9 or YCH terminal
2985,3102 (B,) 3014, 3101 (A,)
2950, 3067 ( A ) 3012, 3067 (A’)
3058 (B,)
3025
YCH central
3014 (4) 3056 (B,)
2929 (A”) 3012 (A’)
3058 (B,)
3025
XCHz Or xCH terminal
XCH
ycc ycc
on Ni(100) is a metallocycle of C4H3stoichiometry, tilted at an angle from the surface. The carbon skeleton should be similar to a cis-butadiene as the one seen in the iron complex listed in Table 111, and the vibrational spectrum should then be composed of modes due to the inner hydrogen atoms (similar to those for cis-butadiene), modes corresponding to a terminal H (analogous to those in the dichlorobutadiene), and vibrations of the carbon chain, which remain almost unchanged with hydrogen isotopic substitution. Our final assignment of HREELS peaks is given in Table 11; it has also been facilitated by the fact that H-D scrambling within the a-deuteriated fragment is negligible, as it has been clearly demonstrated for ethylene on the same surface,8 and is corroborated here by the striking similarities between spectra obtained from normal and a-deuteriated thiophene. After heating above 500 K, few changes in the HREEL spectra take place. The C-H stretching frequency shifts to higher values (to 3050 cm-’), and the peak at 850 cm-’ moves to 750 cm-’. These are signs of increasing unsaturation due to further dehydrogenation of the surface moiety. The carbon-carbon stretching at 1340 cm-’ disappears leaving only the peak at 1450 cm-’. The C-H deformation mode a t 1070 cm-’ shifts to 1150 cm-l. A metallocycle with only &hydrogens (C4HJ could be present at this stage. Additional peaks due to background CO adsorption are observed. They are very hard to avoid because of the time required to acquire the HREELS data. Total dehydrogenation occurs by heating above 650 K. Schoofs et al. have done an infrared study of the chemisorption of thiophene on Ni(lll).14 They report that polymerization of thiophene occurs on this surface. We did not observe peaks in our TDS that we could assign to (14) Schoofs, G. R.; Preston, R. E.; Benziger, J. B. Langrnuir 1985 1,
313.
(trans, C2h) 703 (A,) 746 (BE) 913 (B,) 932 (A,)
850
long-chain hydrocarbons. They also report a sp3 rehybridization that supports their conclusion, but we do not have the needed resolution in HREELS to make similar claims, and the rest of the vibrational spectra can be explained without proposing the formation of any high molecular weight products. Detailed studies on the chemisorption of thiophenes have also been reported on Pt(ll1) and on Mo(100). Chemisorption over Pt(ll1) surfaces is molecular at low temperatures, and the C-S bonds are broken only above 290 K.293 A metallocycle is then formed, of C4H4stoichiometry, bonded tilted with respect to the surface. Dehydrogenation does not occur until heating to 410 K, as seen by TDS experiment^.^ The situation for Mo(100) is slightly more complicated because at least two decomposition pathways operate simultaneously, and the relative importance of each one depends on the initial coverage.’ The predominant mechanism at saturation involves the formation of a metallocycle of C4Hzstoichiometry between 450 and 550 K. It seems that C-S precedes the C-H bond breaking in all cases, but the final stoichiometry of the stable intermediate depends on the dehydrogenation activity of the metal used as a substrate. In conclusion, our TDS and HREELS results for thiophene on Ni(100), together with previously reported NEXAFS and SEXAFS data, lead us to propose that thiophene chemisorption at 90 K is accompanied by C-S bond breaking in the first layer. Multilayer and second layer desorption occurs at 145 and 175 K, respectively, leaving partially decomposed thiophene behind. A fourcarbon metallocycle is formed between 230 and 500 K, with three H atoms and bonded at an angle from the surface. Complete decomposition takes place at 650 K by further dehydrogenation and possibly going through a C4Hz moiety. Registry No. Ni, 7440-02-0; thiophene, 110-02-1.
