Langmuir 1992,8, 2461-2472
2461
Potassium-Oxygen Interactions on a Ru(001) Surface J. Hrbek,*p+T. K. Sham,$M.-L. Shek,s and G.-Q. Xu11 Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7, Canada, and NSLS, Brookhaven National Laboratory, Upton, New York 11973 Received January 2,1992. In Final Form: July 22,1992
The interaction of potassium with oxygen has been examined using synchrotron-based photoemission and NEXAFS, thermal desorption, work function measurements, and isotope exchange. Potassium coverageson Ru(001)surface rangingfrom monolayer to multilayer were investigated. Oxygencoadsorbed with potassium at 80 K forms a potassium-dioxygen complex, where both peroxide and superoxide ions were identified. The complexhas high thermal stability on the Ru(001)surface,decomposinganddesorbing at T > 900 K. Introduction The surface reactions of alkali metals with oxygen have been studied recently quite extensively by various surface techniques, including photoemission, thermal desorption, and vibrational spectroscopies.l-1° These studies have been motivated by the applications of alkali metal compounds in catalysis and in photocathodes and by the attempts to improve the fundamental understanding of oxidation of free-electron metals. Our interest in the interaction of oxygen with potassium originates from our continuing attention to the catalytic properties of a K promoter,3J19 which is believed to be present on the surface of a working catalyst in the form of alkali metal compounds, e.g. alkali metal oxides. Alkali metals have the unique ability of reacting with oxygen to form a variety of oxides, such as the oxide, suboxide, peroxide, and superoxide.11J2 The majority of surface experiments reported so far has dealt with POtassium, mainly because of its technological importance in industrial catalytic reaction such as Fischer-Tropsh synthesis, hydrogenation of nitrogen, and coal gasification. Potassium adsorbed on transition metal surfaces at monolayer coverages interacts with oxygen to form a potassium-dioxygen molecular complex, whose electronic and molecular structures on Ru(001) were reported
recently,4~~J-~~J~ although the complex is yet to be identified unambiguously. In this work the interaction of adsorbed potassium with oxygen on Ru(001) was investigated using work function ($4 measurements, thermal desorption spectroecopy(TDS), isotopic exchange reactions, synchrotron-based valence and core-level photoemission spectroscopies (PES),and near edge X-ray absorption spectroscopy (NEXAFS). We have studied the stepwise oxidation of potassium in the monolayer and multilayer coverage range at low temperature (80 K)and the thermal stability of the potassiumdioxygen products. Both potassium peroxide and potassium superoxide as well as potassium oxide in early stages of coadsorption have been identified as surface species.
Experimental Section Experimentswere performedin three ultrahigh vacuum (UHV) systems, all of which have base pressures below 2 X 10-loTorr. These systems were equipped with standard surface characterization and preparation tools, i.e., LEED (low energy electron diffraction),AES (Augerelectron analyzer),a quadrupole mass spectrometer, a sputter gun, and a potassium doser. The same crystal and sample manipulator were used for PES, @,and TDS measurements; NEXAFS measurements have been carried out on a different crystal.13 The work function,thermal desorption,and isotopic exchange studieswere done in a UHV chamber equippedwith multiplexed quadrupole mass spectrometer (QMS),Kelvin probe, AES, and LEED. The TDS measurementswere carried out with the crystal t Chemistry Department, Brookhaven National Laboratory. facing a differentially pumped, apertured, quadrupole mass * Department of Chemistry, University of Western Ontario. ~pectr0meter.l~ Photoemission experimentswere carried out at NSLS, Brookhaven National Laboratory. the U1 and U14 stations of the Vacuum Ultraviolet (VUV)ring Presentaddress: Department of Chemistry,National University of the NationalSynchrotronLight Source (NSLS). The U1beam of Singapore, Singapore 0511. line is equipped with an extended range grasshopper monochre (1)Patterson, L.-G.; Karleson, S.-E. Phys. Scr. 1977,16,426. (2)Su,C.Y.;Lindau,I.;Chye,P.W.;Oh,S.-J.;Spicer,W.E.J.Electronmator and a UHV samplechamberoutfittedwith a hemispherical Spectrosc. Rel. Phenom. 1983,31,221. analyzer, a partial yield detector, QMS, and LEED. The U14 (3)(a) Shek, M.-L.; Phan, X.;Strongin, M.; Ruckman, M. W. Phys. beam line is equipped with a plane grating monochromatorand Reu. B 1986,34,3741.(b)Shek,M.-L.; Hrbek, J.; Sham,T. K.; Xu,G.-Q. a UHV chamber furnished with a double-passCMA, QMS, and J. Voc. Sci. Technol. A 1991,9,1640. LEED. (4)dePaola, R. A.; Hoffmann, F. M.; Heskett, D.;Plummer, W. J. Chem. Phys. 1987,87,1361. The Ru(001)crystalwas cleaned by a combinationof sputtering (6) (a) Kiakinova, M. J. Vac.Sci. Technol. A 1987,5852.(b) Surnev, and oxygenadsorption-desorption cycles. Potassiumoverlayers L.: RaMelov. G.: Kiskinova. M. Surf. Sci. 1987.179.283. were prepared in situ on a Ru(001)surface by evaporation from '(6) worat&hek,B.; Sessehann, W.; Kuppers; J.; Erlt, G.; Haberland, a well-degassed SAES getter source followed by appropriate H. J. Chem. Phys. 1987,86,2411. annealing.15 Potassium coverages were determined from TDS, (7) Hrbek, J. Surf. Sci. 1986,205,408. (8)E.Bertel, E.; Netzer, F.P.; Rosina, G.; Saalfeld, H. Phys. Reu. B photoemission, and LEED data and referenced to a calibration 1989,39,6082. provided by a ( d 3 X d 3 ) R 3 0 ° LEED structure with a known (9) Hrbek,J.;Xu,G.-Q.;Sham,T.K.;Shek,M.-L.J. Vac.Sci.Techno1. A
.. 1989. - - - - I
7. 2013. - I
(10)Rocker, G. H.; Huang, C.; Cobb, C. L.;Redding, J. D.;Metiu, H.; Martin, R. M.Surf. Sci. 1991,250, 33. (11)Cotton, F.A.;Wilkinson, G. Aduanced Inorganic Chemistry,4th ed.; J. Wiley Q Sons: New York, 1980. (12)Simon,A. In Structure and Bonding of Alkali Metal Suboxides. Structure and Bonding; Springer: New York, 1976;Vol. 26.
(13)Hoffmann, F. M.; Weisel, M.; Eberhardt, W.; Fu, 2.Surf. Sci. 1990,234,L264. (14)Hrbek, J.; Shek, M.-L.;Sham, T. K.; Xu,G.-Q. J . Chem. Phys. 1989,91,5186. (15) dePaola, R. A.; Hrbek, J.; Hoffmann, F.M. J. Chem. Phys. 1986, 82,2484.
0743-7463/92/2408-2461$03.00/00 1992 American Chemical Society
Hrbek et al.
2462 Langmuir, Vol. 8,No.10,1992
coverage of 0.33, relative to the surface atom density of the Ru(001) plane. For the rest of our discussion we will define this coverage as 1 ML (monolayer) of potassium. Oxygen was coadsorbed by backfillingthe vacuum chamber. The crystal was kept at liquid-nitrogentemperature for the potassium deposition, oxygen coadsorption, PES, NEXAFS, and 4 measurements.
5t q
Results and Discussion - 4 1 ML K The studies of adsorption of alkali metals on metals and semiconductors were the first necessary steps that paved the way for coadsorption work from which further studies of coadsorption can be launched.I6 The fact that a potassium-ruthenium system is one of the most studied systems is not surprising, considering the catalytic applications of ruthenium as a hydrogenation catalyst and potassium as a promoter of the reaction. There is a reasonable consensus among several inde0 5 10 15 pendent laboratories on the experimental data for K/Ru(OOl);16J7J8 however, the conventional interpretation Oxygen Exposure (L) employing the notion of charge donation of the outer s Figure 1. Work function 4 as a function of oxygen exposure to electrons of the adsorbed alkali metal to the conduction a clean Ru(001) surface and to Ru(001) covered by 1, 3, and 6 band of the substrate, which has been around for many ML K. The 4 was monitored continuously using a Kelvin probe is now being challenged by t h e o r i ~ t aand ~ ~ ? ~ ~ ( A 4 Electronics GmbH) with a surface held at 85 K. The arrows expermentalist~.24>~5 The nonionic bonding picture with mark the work function 4 of a clean Ru(001) and polycrystalline little if any charge transfer to the substrate was, however, K. contested quite recently.26 Work Function Measurements. The lowering of the (2) The coplanar arrangement of potassiumand oxygen work function and of the adsorption energy with increasing ions.219 The depolarization of coadsorbed species as a coverage is the typical behavior of alkali metals on metallic function of oxygen coverage was modeled and shown to surfaces.16*21Coadsorptionof oxygen with potassium (OK describe the experimental 4 fairly well.3o It was also 1 1ML) on a Ru surface leads initially to a decrease of suggested that the 4min is characteristic of the transition the 4 followed by an increase and saturation. Figure 1 between metallic and ionic states of coadsorbed potassidisplays this behavior for three different K coverages, and um,4 since the structural changes are indicated by LEED comparison is made with the 4 of a clean Ru exposed to data for the K-O stoichiometries around the 4- for oxygen. The 4 of clean K and Ru surfaces are marked by potassium monolayer. the arrows. Notice a shiftof the 4 minimum toward higher It is clear from Figure 1 that a qualitatively similar oxygen exposures with increasing initial coverage of behavior is observed for all potassium coverages investipotassium, higher oxygen exposure needed for the satugated in this study. Higher oxygen exposures required to ration, and lower 4 at saturation. reach the 4minfor thicker potassium layers manifest the influence of bulk diffusion processes. The stepwise There is no general agreement on how to interpret the formation of K-0 species with fixed (although unknown) 4 measurements,even in a monolayer range of coverages. stoichiometry may be suggested, where the first oxidation Two models were proposed to explain the observed 4 data stage must be reached throughout the potassium film in monolayer range coverages: before the oxidation can proceed further. The similarity (1)Double layer formation, where chemisorbed oxygen between monolayer and multilayer behavior implies the adatoms diffuse under the alkali metal layer.27 The identical physical process governing the observed pheresulting positive outward dipoles lead to a decrease of 4. nomenon. After saturation of subsurface sites, the top sites become We can therefore conclude that the double layer occupied with the negatively polarized oxygen adatoms mechanism with oxygen penetrating below the surface is whose dipoles point in the opposite direction, thus more fitting for the processes of potassiumoxidation than counterbalancing the initial 4 drop. A phenomenological the coplanar arrangement mechanism. The quantitative modeP8 based on the oxygen incorporation under the interpretation of the coadsorbate induced 4 should take surface layer describes the 4 behavior quite well. into account not only the morphological changes within the surface layer but also the changes in the chemical and (16) Bonzel, H. P. Surf. Sci. Rep. 1987,8, 43. (17) Weimer, J. J.; Umbach, E.; Menzel, D. Surf. Sci. 1985,155, 132. electronic properties of surface species. (18)Madey, T.; Benndorf, C. Surf. Sei. 1985,164,602. LEED. The only LEED pattern of the coadsorbed 0/1 (19) Langmuir, I. J. Am. Chem. SOC.1932, 54, 2798. ML K was observed at the 4 minimum and has been (20) Gurney, R. W. Phys. Reu. 1935,47, 479. (21) Ertl, G. In Physics and Chemistry of Alkali Metal Adsorption; described previ~usly.~JAt higher oxygen exposures, as Beonzel, H. P., Bradshaw, A. M., Ertl, G.; Eds.; Elsevier: Amsterdam, well as for higher 6 ~the , coadsorbed layers are disordered. 1989; p 1. The rest of this paper will deal with experimental results (22) Wimmer, E.; Freeman, A. J.; Hiskes, J. R.; Karo, A. M. Phys. Reu. E 1989,2%, 3074. and their interpretation at three potassium coverages. One (23) Ishida, H. Phys. Rev. E 1989, 39, 5492. monolayer of potassium ((d3Xd3)R30° LEED pattern) (24) Riffe, D. M.; Wertheim, G. K.; Citrin, P. H. Phys. Reu. Lett. 1990, was examined for reference purposes, as most coadaorption 64.. 571. studies were until now carried out on this system. (25) Modesti, S.;Chen, C. T.; Ma, Y.;Meigs, G.;Rudolf, P.; Sette, F. Phys. Reu. B 1990,42,5381. However, because of the catalytic relevance of alkali metal (26) (a) Derby, G. P.; King, D. A. Faraday Discuss. Chem. SOC.1990, with higher initial coverges (actual potassium loading of 89,259. (b) Benesh, G . A.; King, D. A. Chem. Phys. Lett. 1992,191,315. ~~
(27) Papageorgopoulos, C. A.; Chen, J. M. J. Vac. Sei. Technol. 1972,
9. 570. _, -._.
(28) Parker, S. D.; Rhead, G. E. Surf. Sci. 1986,167, 271.
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(29) Pirug, G.; Broden, G.; Bonzel, H. P. Surf. Sci. 1983,94, 323. (30) Albano, E. V. Appl. Surf. Sci. 1983, 14, 183.
Langmuir, Vol. 8, No.10, 1992 2463
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