J. Phys. Chem. 1992, 96,790-795
790
or well-known binary semiconductors, this factor has not been emphasized so far. Further investigations are necessary to increase the understanding of the interrelation of electronic structure with semiconductor surface reactivity on a microscopic basis. Acknowledgment. The helpful discussions with Dr. D. Schmeisser, Dr. C. Pettenkofer, and Prof. J. K. Sass are gratefully
acknowledged. We thank Dr. S. Fiechter for his help with the thermodynamic calculation program. Prof. K.Bachmann and Dr. P. Lange kindly provided the single crystals for these experiments. The work was partly supported by the BMFT'.
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12018-95-0;
Heterocyclization Sites on the Sulfided Pd( 111) Surface Andrew J. Gellman Department of Chemistry, University of Illinois. Urbana, Illinois 61801 (Received: March 7, 1991)
We have examined the heterocyclization of acetylene (C2Hz) to thiophene (C,H,S) on the sulfided Pd( 1 1 1 ) surface. This reaction has been found to occur at defects in the sulfided surface which can be produced either by annealing the surface to temperatures >800 K or by sulfidingthe surface to near-saturation. At 300 K both thiophene and CO will adsorb selectively at these defects and can be used to titrate the surface defect concentration. The yield of thiophene from the acetylene heterocyclization reaction is correlated with the defect concentration as measured by these methods and, hence, it appears that the reaction occurs at defects in the sulfided surface. Vibrational spectra indicate that CO adsorbed at these defects is bound in a bridging configuration between Pd atoms indicating that the defects expose the metallic substrate.
1. Introduction Many surface chemical processes are known or thought to occur at defects on otherwise crystalline solid surfaces. This is true of many reactions catalyzed by ionic solids such as metal oxides and metal sulfides on which the active sites are thought to be anion vacancies.*J While the surface science community has made great progress in understanding the details of surface-induced reactions on many crystalline metallic surface, there has been much less success in the study of chemistry catalyzed by surface defects. This is particularly true of our understanding of the surface chemistry of compound materials. The notable exception to these statements is the work that has made use of high Miller index surfaces of metal crystals which expose high densities of steps and steps with kinks in ordered array^.^ Unfortunately it has not been generally possible to cut and characterize high Miller index surfaces of most solid compounds. The difficulties associated with the study of defect catalyzed reactions arise from the need to identify chemistry as occurring at defects as opposed to the ordered planes or terraces of a crystalline surface. In most cases defects are present in low concentrations which are difficult to control. This is exacerbated by the inherently low sensitivity of many of the analysis techniques common to surface science investigations. In this paper we describe the formation of defects in a sulfided Pd( l l l ) surface by either annealing the surface or allowing the surface sulfur coverage to come arbitrarily close to saturation. We have found methods to titrate the surface for defect concentration and to perform some limited characterization of these defects using CO adsorption. Furthermore, it has been possible to observe surface chemistry which can be ascribed directly to these defects. The surface chemical reactions which we have identified as occurring at defects on the sulfided Pd( 1 1 1 ) surface involve heterocyclization and cyclotrimerizationof acetylene, desorption of CO adsorbed at T > 300 K, and desorption of adsorbed thiophene at T > 300 K.4 The reaction which we have focused ~~
(1) (a) Yamazaki, Y.; Kawai, T. Adu. Catal. 1980, 229. (b) Massoth, F. E.Adv. Carol. 1978, 27, 265. (2) Chianelli, R. R. Caral. Rev. Sci. Eng. 1977, 26 (3), 361. (3) (a) Somorjai, G.A. Adv. Card 1977, 26, 2. (b) Davis, S. M.; Somorjai, G. A. The Chemical Physics of Solid Surfaces and Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: New York, 1982; Vol. 4.
0022-3654/92/2096-790$03.00/0
on is one in which adsorbed acetylene heterocyclizes with sulfur atoms to yield thiophene which can then desorb into the gas phase.5 This reaction is similar in many respects to acetylene cyclotrimerization to benzene which has been observed and studied on the clean Pd( 1 1 1) surface.6 The heterocyclization of acetylene to thiophene is observed during desorption experiments following acetylene adsorption on the sulfided Pd(ll1) surface at low temperatures (lo* Torr." Saturation of the Pd( 111) surface with sulfur at 700 K generates a surface with a (d7X.\/7)R19° lattice which is incapable of ~~~
(1 1) Bradshaw, A.
M.;Hoffmann, F. M. Surf. Sci. 1978, 72, 513.
794 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
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Figure 8. HREELS vibrational spectrum of CO adsorbed on (A) Pd(1 11) surface sulfided to saturation at 600 K (T,Q = 130 K) (Zel = 1.0 X IO5counts/s, fwhm = 65 an-'),(B)sulfided Pd( 111) surface annealed to 1000 K (Tab = 130 K) (Zel = 1.1 X los counts/s, fwhm = 65 cm-I), and (C) sulfided Pd( 111) surface annealed to lo00 K (Tad,= 300 K) (4 = 1.3 X lo5 counts/s, fwhm = 55 cm-I), 15 s/point.
adsorbing CO at room temperature. Adsorption of CO at 120 K results in an overlayer whose vibrational spectrum is shown in Figure 8A and consists of molecules bound in both bridging and linear sites. As shown above, annealing the surface to 1000 K for 100 s generates defects or sites at which CO can adsorb with a higher heat of adsorption than on the completely sulfided surface. More importantly, the HREEL spectrum in Figure 8B shows an increase in the contribution of CO bound in bridging sites on the surface. Exposure of the surface to CO at 300 K results in selective adsorption of CO at the reaction sites. The HREEL spectrum in Figure 8C reveals, as expected, that this species is indeed bound exclusively in a bridging configuration. The small contribution from CO bound linearly may well be due to CO adsorbed from the background during the time (>1 h) necessary to collect the spectrum at low temperature. Closer inspection of the spectra in Figure 8 reveals that not only is the CO bound at bridging sites selectively but its vibrational frequency (1890 cm-I) is clearly lower than that of CO bound in the bridging sites on the fully sulfided surface (1955 cm-I). It is also lower than the frequency of CO adsorbed at bridging sites on the clean surface at either 300 or 120 K (1940 cm-I). Furthermore, this feature in the HREEL spectrum is clearly much broader than either of the loss features on the clean or fully sulfided surfaces and is much broader than the elastically scattered peak indicating quite a high degree of inhomogeneity in the defect sites.
4. Discussion 4.1. HeterocyclizationSite Formation. The primary aim of this paper is to demonstrate that the heterocyclization of acetylene on the sulfided Pd( 111) surface is occurring at defect sites on this surface. The secondary aim has been to characterize these defects by examination of CO which can be selectively adsorbed at these sites. It is quite clear from the data illustrated in Figures 5 and 6 that the heterocyclization sites on the sulfided Pd( 111) surface with a ( d 7 x ~ ' 7 ) R 1 9overlayer ~ lattice exist at sulfur coverages near saturation. As the sulfur coverage is increased from zero by adsorption and thermal decomposition of H2S,heterocyclization sites are formed. The density of reaction sites increases until the sulfur coverage reaches -0.9Osat, at which point it begins to decrease until at saturation there is no heterocyclization activity observed. At this point cyclization activity can be regenerated by annealing of the surface for periods of 100 s at temperatures in the range 800-1 100 K. Annealing is accompanied by loss of up to 10% of the sulfur from the surface. The heterocyclization
Gellman sites are referred to as defects simply because it is clear that they represent a minority of the crystal surface area. The direct correlations between CO adsorption at 300 K, acetylene cyclotrimerization to benzene, and acetylene heterocyclization to thiophene suggest that all three processes are occurring at the same sites on these surfaces. The yields plotted in Figures 5 and 6 have each been scaled by a single constant to fit the same curve. It is important to note that our estimates of the absolute yields of the heterocyclization reaction are quite crude but indicate that the maximum yield of thiophene from the surface corresponds to roughly 0.1% of a monolayer where the monolayer is the coverage at which thiophene saturates the sulfided Pd( 111) surface without forming a multilayer. Our estimate of the CO coverage adsorbed at the defects is somewhat better in that it can be compared with the saturation coverage adsorbed on the clean surface at room temperature (0.5 ML). This indicates that on a surface annealed to lo00 K the defects are able to adsorb roughly 5% of a monolayer of CO at 300 K. The apparent discrepancy between the amount of CO adsorbed at defects and the amount of thiophene produced at the defects results from the fact that the heterocyclization reaction is merely one of a number of reaction pathways available to adsorbed acetylene. Others include cyclotrimerization to benzene, desorption into the gas phase, and self-hydrogenationof acetylene to ethylene. In fact, the heterocyclization reaction consumes only a small fraction of the adsorbed acetylene. Although they have not been studied in detail in this work, the kinetics of defect sites formation are quite interesting and it is worth making some mention of them at this point. As mentioned above the density of surface defects, as measured by CO adsorption, is primarily dependent upon the temperature to which the surface is annealed rather than the duration of the annealing treatment. This is true in the sense that annealing for periods in excess of 100 s does not increase the defect site density appreciably. For short annealing periods the defect density must be time dependent; however, on time scales of >lo0 s it appears that the defect density approaches some irreversible equilibrium value. The equilibrium is irreversible in the sense that cooling the crystal from the annealing temperature down does not result in a decrease in the defect density. At this point it is not clear what the driving force for defect formation is except that the kinetic barrier for their formation is obviously dependent on the defect concentration. The exact process by which sulfur is lost from the surface is unknown other than to say that no desorption of sulfur is observed during heating. It is possible that sulfur is dissolving into the crystal bulk which certainly should be an irreversible process. 4.2. Defect Site Characterization. At this point we have only been able to characterize the defects formed on the sulfided surface by examination of CO adsorption/desorptionand by measurement of the vibrational spectrum of CO selectively adsorbed at these defects. The structure of the ( ~ ' 7 X d 7 ) R 1 9 ~sulfide overlayer that we are dealing with is unknown at this point. CO adsorbs on this surface with a heat of adsorption of 12 kca1/mol,l2 which is much lower than the 34 kcal/mol measured on the clean surface?J0 Comparison of the CO desorption spectra indicates that the saturation coverage of CO on the sulfided surface is slightly lower than on the clean Pd( 111) surface (0.47 ML). The surface unit cell of the (d7Xd7)R19° lattice contains seven Pd atoms and hence the nominal coverage of CO is three molecules per unit cell. The vibrational spectrum of CO on the clean surface at 300 K indicates that the molecule is adsorbed in a bridging configuration between Pd atoms with a vibrational frequency of 1940 cm-I. When adsorbed on the clean surface at low temperatures, it is possible to form a compressed overlayer with a coverage in excess of 0.50 and some of the adsorbed CO molecules are forced to bind linearly with single Pd atoms. These have CO vibrational frequencies of 21 10 cm-'. On the sulfided surface it is clear that CO occupies both linear and bridging sites within the unit cell. The stretching frequencies of molecules bound in the linear and (12) Redhead, P. A. Vacuum 1962, 12, 203.
Sulfided Pd( 11 1) Surfaces bridging configurations are 1955 and 21 15 cm-I, respectively, which, to within our resolution, are the same as observed on the clean surface. Annealing of the surface to generate heterocyclization sites also generates sites at which CO can adsorb on the surface at 300 K. CO desorbs from these sites in the temperature range 300-400 K indicating a heat of adsorption of 18-22 kcal/mol.I2 This is still lower than the value observed for the clean surface indicating that the sites created are not actually open regions of clean Pd( 111) surface. The peak is fairly broad and clearly does not have the shape expected for a first-order process indicating either that there are strong intermolecular interactions between adsorbates or that there is some degree of heterogeneity to the surface. Examination of the vibrational spectrum of CO adsorbed on an annealed surface at low temperature indicates that there is an increase in the fraction of molecules adsorbed in bridging configurations. The vibrational spectrum of CO bound selectively at the defect sites by adsorption at 300 K indicates that all these molecules are bound to the metal in a bridging configuration. Clearly the loss of surface sulfur results in exposure of Pd atoms and the structure of the defects is such that the spacing between exposed substrate atoms is sufficiently small that CO molecules can bind between pairs of these atoms as on the clean surface. Closer examination reveals that the CO stretch mode of these molecules is centered at 1890 cm-] with a width of 120 cm-I (full width at half maximum, fwhm). The position of this peak is significantly lower than that observed for CO on the clean surface at saturation (1940 cm-I). Since the fwhm of the elastically scattered peak is 55 cm-' and the width of the linearly bound CO on the sulfided surface is 70 cm-I, it appears that there is some degree of heterogeneity in the defects. The shift of the peak to frequencies below those normally observed for CO in a bridging site on either the clean or the sulfur-saturated surface is also noteworthy. On the clean surface the CO stretching frequency for molecules bound in the bridging sites is actually observed at a range of frequencies. At coverages below 0.33 ML the band is found at 1830 cm-' and is attributed to CO bound in a 3-fold hollow. As the coverage is increased from 0.33 to 0.50 ML, the peak position moves continuously up to 1946 cm-I. This effect has been observed using both HREELS and IR absorption spectroscopies and is attributed to a change in adsorption site resalting in the formation of a bridge bonded species.11v13 We observe that CO adsorbed selectively at defects (13) Timbrell, P. Y . Ph.D. Thesis, Cambridge University, 1987.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 195 on the surface has a vibrational band at 1890 cm-l. This is still in the range normally associated with adsorption of CO in a bridging configurati~n'~ but is at much lower frequency than that of CO adsorbed to saturation coverage in bridging sites on either the clean or sulfur-saturated surface. This is consistent with the previous suggestion that the defect sites on the surface are not just areas of exposed Pd( 111 ) surface. The width of the CO vibrational band and the width of the desorption features of both CO and thiophene bound selectively at defects indicate that there is some degree of heterogeneity to their structure. The position of the vibrational band indicates that interaction between CO molecules is not nearly as great as in the monolayer formed on the clean Pd(ll1) surface and hence that the local density of CO molecules adsorbed at these defects is not as great as in the 0.50 ML overlayer that can be generated on the clean surface. On the basis of the data presented here, it is really only possible at this point to describe the defects as sites at which Pd atoms have been exposed. 5. Conclusions We have shown that the heterocyclization reactions on the sulfided Pd( 111) surface cccur at defects in the sulfide film which can be generated either by annealing at temperatures >800 K or by sulfiding the surface to coverages close to saturation. These defects are the result of loss of sulfur from the surface during annealing and can be titrated by CO which, at 300 K, will adsorb selectively at these sites. There is a direct correlation between the density of defects as determined by CO adsorption and the yield of the acetylene heterocyclization and cyclotrimerization reactions on these surfaces. Examination of the vibrational spectrum of CO on the surface indicates that the defects are sites at which Pd atoms are exposed and CO can bind in a bridging configuration.
Acknowledgment. This work was supported by the Department of Energy under Grant No. DE-AC02-76ER01198through the Materials Research Lab of the University of Illinois. A.J.G. holds a David and Lucile Packard Foundation Fellowship in Science and Engineering and is an A. P. Sloan Foundation Research Fellow. R@try NO. H-H, 74-86-2; Pd, 7440-05-3; CO, 630-08-0; thiophene, 1001-02-1. (14) Ibach, H.; Mills, D. Electron Energv Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982.
