Langmuir 1989, 5 , 348-352
348
micelle first appears a t the left side of the cusp of the mixture cmc curve. Both fluorocarbon-rich and hydrocarbon-rich micelles, which are mutually saturated each other, coexist within the second cmc curves. These second cmcs were also observed by self-diffusion measurements of the Fourier transform NMR pulsed-gradient spin-echo method (FT-PGSE NMR).39
micellar systems. The significant changes in the fluorescence and NMR spectra were observed above the mixture cmc. The abrupt changes far above the mixture cmc were assigned to the formation of another type of mixed micelles, that is, the second cmc. Above the second cmc, both fluorocarbon-rich and hydrocarbon-rich mixed micelles coexisted.
Conclusion The use of probes has been investigated to determine the second cmc and to examine the microenvironment of
Acknowledgment. We are grateful to Dainippon Ink Chemical Industry Co., Ltd., for providing the fluorocarbon surfactant. Registry No. LiFOS, 29457-72-5; LiDS, 2044-56-6; LiTS, 52886-14-3;ANS, 82-76-8;auramine, 2465-27-2;pyrene, 129-00-0.
(39) Asakawa, T. et al., unpublished data.
Influence of Pre- and Postdeposited Au on Coadsorbed CO on Ru(001) Kyoichi Sawabe, Chikashi Egawa, Tetsuya Aruga, and Yasuhiro Iwasawa" Department of Chemistry, Faculty of Science, T h e University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received August 25, 1988. I n Final Form: November 18, 1988 The geometrical and electronic effects of pre- and postdeposited Au on the adsorption of CO on Ru(001) have been examined by means of AES, TDS, UPS, XPS, and LEED. It is shown that Au grows via the Stranski-Krastanov mechanism on the clean Ru(001) surface held at 670 K as previously reported. Submonolayer coverages of Au form two-dimensional (2D) islands at around 670 K, and annealing this surface to 1000 K causes the nearly atomical dispersion of Au atoms over the surface as characterized by sharpening of Au 5d features in UPS. The 2D islands of Au suppress the CO uptake without any noticeable change in the shape of CO TD spectra, whereas highly dispersed Au atoms effectively prevent the formation of long-range order of CO, hence resulting in remarkable broadening of CO TD peaks. The postdeposition of Au on the CO-saturated surface results in extensive crowding of CO, which is characterized by unusually large binding energies of CO 5 u / 1 ~and 4u molecular orbitals and considerable lowering of CO desorption temperatures. The effects of postdeposited Au are explained in terms of increased CO-CO interaction and weakened CO-metal bonds due to the enhanced crowding by Au. Thusthe influence of pre- and postdeposited Au on coadsorbed CO on Ru(001) is well ascribed to the geometrical (ensemble)effect. There is no indication of the electronic effect of Au in the present system.
Introduction Recently, bimetallic surfaces prepared on single crystals have been extensively investigated toward a goal of the full understanding of the mechanism of enhancement of catalytic activity and/or selectivity in bimetallic catalysts relative to their individual components. Several models have been hypothesized to explain the drastic change of catalytic properties in bimetallic systems. First, the alloying may cause an electronic modification of either or both of the component metals (ligand effect). Second, the additive metal may geometrically block the formation of active sites or ensembles which are required for a reaction to occur (ensemble effect). Third, each component may promote different reaction steps or cooperatively enhance the reaction and, thus, act synergistically with each other (synergistic effect). Among many possible combinations for bimetallic systems, the combination of group Ib/group VI11 metals has been most widely investigated by using well-defined single-crystal s u r f a ~ e s . l - ~The group Ib metals have a dl0s1 (1) Christmann, K.; Ertl, G.; Shimizu, H. J . Catal. 1980, 61, 397. (2) Vickermann, J. C.; Christmann, K.; Ertl, G. J . Catal. 1981, 71, 175. (3) Vickermann, J. C.; Christmann, K.; Ertl, G.; Heimann, P.; Himpsel, F J.; Eastmann, D. E Surf. Sci. 1983. 134. 367.
0743-7463/89/2405-0348$01.50/0
configuration and hence exhibit only very weak interaction with molecules like CO and Hz.Therefore, the addition of group Ib metals such as Cu and Au onto transition-metal surfaces is expected to result only in blocking of adsorption sites for those molecules. Nevertheless, epitaxial and alloyed Au/Pt(lll) surfaces have been reportedg to show the enhancement in activity and selectivity for cyclohexane dehydrogenation to form benzene. It was suggested that Au serves to weaken the chemisorption bond of benzene and hence reduce the selfpoisoning by the adsorbed product. The same A u / P t ( l l l ) sample also exhibited the enhancement in activity for hexane conversion, whereas the deposition of Au onto Pt(100) did not show any promotion effects, suggesting the structure sensitivity of alloy catalysis for this reaction.1° It has also been demonstrated5 (4) Houston, J. E.; Peden, C. H. F.; Feibelman, P. J. Phys. Reu. Lett. 1986, 56, 375. (5) Peden, C. H. F.; Goodman, D. W. J. Catal. 1986, 100, 520. (6) Gocdmann, D. W.; Yaks, J. T., Jr.; Peden, C. H. F. Surf. Sci. 1985, 164, 417. (7) Rocker, G.; Tochihara, H.; Martin, R. M.; Metiu, H. Surf. Scz. 1987, 187 . - ., m ---. (8) Harendt, C.; Christmann, K.; Hirschwald, W.; Vickermann, J. C. Surf. Sci. 1986, 165, 413. (9) Sachtler, J. W. A.; Somorjai, G. A. J . Catal. 1984, 89, 35.
6 1989 American Chemical Society
Influence of Au on CO Coadsorbed on R u
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deposition time/min Figure 1. Variation in Auger intensities of Au and Ru at 670 K as a function of Au deposition time. that a Ru(001) surface covered with a submonolayer quantity of Cu exhibits an enhanced activity toward cyclohexane dehydrogenation. In this case, however, the effect was ascribed to the electronic modification of the constituent metals and/or to the synergistic effect. On the other hand, it has been suggested' that Cu deposited onto a CO-saturated Ru(001) surface interacts directly with oxygen atoms of preadsorbed CO. Thus, the effect of group Ib metal present on group VI11 metal surfaces still remains under debate. In order to clarify the effect of group Ib metals, several mechanisms should be examined separately, which would also be worthwhile for the construction of a general picture of bimetallic catalysis. The present study on the influence of Au on coadsorbed CO on Ru(001) is a part of our bimetal surface to clarify essential factors and elementary steps of the adsorption and catalysis on these surfaces and to establish a solid basis for an atomic-scale surface design. In the course of this study, Harendt et a1.8 reported results of Auger electron spectroscopy (AES)and thermal desorption mass spectrometry (TDS) on the adsorption of CO on an Au-precovered Ru(001) surface. In this study, we have examined their conclusions by means of ultraviolet (UPS) and X-ray (XPS) photoelectron spectroscopies as well as AES, LEED, and TDS. In addition, we have also investigated the influence of postdeposited Au on the preadsorbed CO. All the results are well explained by the geometrical effects of Au on the adsorption state of CO. We believe that the observations described here serve as a standard for similar experimental studies on more complicated systems having strong interactions between additive and substrate metals.
Experimental Section The experiments were performed in two different stainless steel UHV apparatus. One of those was a Vacuum Generators ESCALAB5, in which the UPS measurement was carried out. All the spectra were recorded at normal emission. Another UHV system was equipped with a four-grid retarding field energy analyzer for AES and low-energy electron diffraction (LEED)and a quadrupole mass spectrometer (QMS) for TDS. The Ru(001) sample was cleaned by repeated cycles of Ar ion sputtering and subsequent annealing to 1200 K. The contribution from edges of the sample to TDS spectra was minimized below 1% by sulfur masking in 3 X lo4 Pa of HzS for 2 min followed by Ar ion sputtering of only a front face. The CO (99.99%) gas used was purified over a Pd catalyst in a closed circulating system with a liquid Nz trap, and its purity was checked by the QMS. The (10)Yeates, R. C.; Somorjai, G. A. J . Catal. 1987, 103, 208. (11) Egawa, C.;Aruga, T.; Iwasawa, Y. Surf. Sci. 1987, 188, 563. (12)Egawa, C.;Iwasawa, Y. Surf.Sci. 1988, 195, 43. (13)Egawa, C.; Sawabe, K.; Iwasawa, Y. J. Chem. SOC.,Faraday Trans. 1 1988,84, 321.
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BINDING ENERGY /eV Figure 2. He(I1) UP spectra for Ru(001) surfaces: (a) clean surface, (b) surface covered with ca. 0.5 ML of Au at 300 K, (c) after annealing to 1000 K.
deposition of Au was carried out by resistive heating of a W wire on which a Au droplet was melted.
Results and Discussion Growth of Au on Ru(001). We have first examined the growth of Au on Ru(001). The results of AES and LEED are quite similar to those of Harendt et and are briefly summarized as follows: Figure l a shows typical data for Auger intensities of Au NVV (69 eV) and Ru MNN (281 eV) transitions measured as a function of Au deposition time with the Ru(001) surface held a t 670 K. Each plot of Ru and Au Auger intensities exhibits only one break a t -12 min and then shows a monotonic change (increase for Au and decrease for Ru) with much more gentle slopes. The essential features of the plots were reproducibly observed for various deposition rates. Thus, the growth of Au on the Ru(001) surface appears to proceed via the Stranski-Krastanov mechanism, which refers to three-dimensional (3D) growth on top of an initially formed monolayer, as already suggested.* We define the Au coverage a t 12 min in Figure l a as one monolayer (1 ML). The coverage values given below were determined by using this Auger data as a standard. Annealing the surface covered with submonolayer coverages of Au to 1100 K caused no detectable change in Auger signals of Ru and Au. This indicates that no alloying of Au with the Ru substrate took place, which is in accordance with the fact that Ru and Au do not form any bulk alloys. We also confirmed the LEED observation by Harendt et a1.,8 which indicates the growth of noncrystalline Au islands at low temperatures and the atomic dispersion of Au atoms upon annealing to 1000 K. We have obtained further evidence by UPS for the atomic dispersion of Au upon annealing to high temperatures. Figure 2 shows He I1 U P spectra for the Ru(001) surface covered with ca. 0.5 ML of Au before and after annealing to lo00 K. The deposition of Au on the Ru(001) surface a t 300 K induces broad Au 5d features in UP spectra at binding energies in the range of 2-5 eV below EF (curve b). The shape of the Au 5d features resembles that of Au(lll),14indicating that Au forms 2D or 3D aggregates at 300 K. As shown by curve c in Figure 2, annealing this surface to 1000 K causes the sharpening of Au 5d features, producing well-resolved peaks a t 3.2, 3.5, and 4.8 eV. This is naturally interpreted as due to the decrease in overlap of Au 5d orbitals by breakdown of Au islands into individual atoms. In XP spectra of Au 4f7,, (87.7 eV) (14)Shen, X.Y.; Frankel, D. J.; Lapeyre, G. J.; Smith, R. J. Phys. Reu. 1986, B33, 5372.
350 Langmuir, Vol. 5, No. 2, 1989
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