J. Phys. Chem. 1992, 96, 2265-2271 for the usual NaHY zeolite an acidity lower than for US-Ex, Le., similar as for the SAPO molecular sieves. In fact, the corresponding wavenumbers and force constants for Rh1(C0)2/NaY are at 21 11 and 2045 cm-I and 1745 N m-l, respecti~ely,~-~’ i.e., at nearly the same values as for the SAPO molecular sieves. Including these results the acidity decreases in the order US-Ex > NaY N SAPO-5 N SAPO-11 >> AIPOrl1, in good agreement with the results obtained by other authors with different methods. This correspondence is an additional argument for localizing the (37) Maneck, H.-E.; Gutschick, D.; Burkhardt, I.; Liicke, B.; Miessner, H.; Wolf, U. Caral. Today 1988, 3, 421.
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well-defined dicarbonyls at the cation sites in the vicinity of the isolated A1 (Y zeolite) or Si (SAPO) atoms.
Acknowledgment. We thank Mrs. E. Laffler and Dr. D. Muller for stimulating discussions, Dr. P. Ugliengo for providing the MOLDRAW program, and Mrs. P. Haase for technical assistance. We thank one of the reviewers for his critical remarks regarding the surface acidity of molecular sieves and Dr. A. Sauer for a fruitful discussion on this topic. Financial support by the Bundesminister fur Forschung und Technologie (BMFT 521-53303051014B4) is gratefully acknowledged. Registry No. CO, 630-08-0; Rh,7440-16-6.
Properties of the Ru(OOOl)/Ce-H, Interface: A Model System for Transition-MetaVRare-Earth Hydride Catalysts Andrew P. Walker and Richard M. Lambert* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, UK (Received: August 8, 1991)
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Controlled deposition of Ce onto Ru(0001) results in layer-by-layer growth of the rare-earth metal. At low Ce coverages (600 K,they undergo phase separation, transforming into biphasic CeH,+,/TM materials, which are catalytically active. However, when the same alloys are activated in a CO/H2 gas stream the resulting Ce02/TM systems are catalytically inert toward ammonia generation. Our earlier in situ X-ray absorption spectroscopy study6 of catalysts derived from the alloy CeRu, revealed significant differences in both the morphology and the electronic structure of the Ru entities present in the CeH2+,/Ru and CeO,/Ru systems. It was found that very small Ru species, possibly atomically dispersed, were present in the active CeH2+JRu catalyst. In contrast with this, all the Ru was present as -40-A crystallites in the inactive Ce02/Ru sample. Furthermore, in the former case the very small Ru entities were thought to be electron-rich with respect to the bulklike Ru metal particles present in the oxide-supported system: the properties of the CeH2+,/Ru interface are thus of considerable interest. These findings provide the motivation for the present work and this paper describes the first part of a parallel, coordinated study of the properties of a Ru/Ce/H model system. We have characterized the interaction between metallic cerium and the Ru(0001) single-crystal surface and the interaction of the Ru/Ce system with H2 is reported, as are the properties of Ce/H codeposited films. Subsequent papers will deal with the interaction of CO and O2with Ce thin films on Ru and with pertinent aspects ‘To whom correspondence should be addressed.
of the interaction of CeH, films with CO and 02.
Experimental Section Experiments were carried out in a conventional ultrahighvacuum chamber, equipped with a four-grid RFA system for LEED/Auger analysis, a quadrupole mass spectrometer for temperature-programmed desorption (TPD), and a hot cathode ion source for sputter cleaning. The system base pressure was 2 X 1O-Io Torr. The Ru(0001) single crystal was cut from a rod of 99.99+% purity and was oriented and polished by standard methods. It was mounted in the chamber via molybdenum heating wires and had a Pt/Pt13%Rh thermocouple spot-welded to its edge. The sample was cleaned using a combination of argon ion bombardment and high-temperature oxygen treatments (1 350 K/lO-’Torr of 02/2min) followed by annealing at 1550 K to remove subsurface oxygen species. A movable heat sink permitted sample heating to 1550 K and cooling to 140 K. An electron bombardment source was used to heat high-purity (99.99%) cerium to the temperatures required to generate a useful flux of evaporated metal at the Ru crystal surface; film deposition was carried out a t pressures of
