Surface chemistry of NO and NO2 on the Pt(1 1 0)-(1 × 2) surface: A comparative study

The surface chemistry of NO and NO₂ on clean and oxygen-precovered Pt(1 1 0)-(1 × 2) surfaces were investigated by means of high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). At room temperature, NO molecularly adsorbs on Pt(1 1 0), forming linear NO(a) and bridged NO(a). Coverage-dependent repulsive interactions within NO(a) drive the reversible transformation between linear and bridged NO(a). Some NO(a) decomposes upon heating, producing both N₂ and N₂O. For NO adsorption on the oxygen-precovered surface, repulsive interactions exist between precovered oxygen adatoms and NO(a), resulting in more NO(a) desorbing from the surface in the form of linear NO(a). Bridged NO(a) experiences stronger repulsive interactions with precovered oxygen than linear NO(a). The desorption activation energy of bridged NO(a) from oxygen-precovered Pt(1 1 0) is lower than that from clean Pt(1 1 0), but the desorption activation energy of linear NO(a) is not affected by the precovered oxygen. NO₂ decomposes on Pt(1 1 0)-(1 × 2) surface at room temperature. The resulted NO(a) (both linear NO(a) and bridged NO(a)) and O(a) repulsively interact each other. Comparing with NO/Pt(1 1 0), more NO(a) desorbs from NO₂/Pt(1 1 0) as linear NO(a), and both linear NO(a) and bridged NO(a) exhibit lower desorption activation energies. The reaction pathways of NO(a) on Pt(1 1 0), desorption or decomposition, are affected by their repulsive interactions with coexisting oxygen adatoms.     

Photon-induced chemical vapor deposition of molybdenum on Pt(1 1 1)

Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption and photon-induced decomposition of Mo(CO)₆. Mo(CO)₆ adsorbs molecularly on a Pt(1 1 1) surface with weak interaction at 100 K and desorbs intact at 210 K without undergoing thermal decomposition. Adsorbed Mo(CO)₆ undergoes decarbonylation to form surface Mo(CO)_x (x ? 5) under irradiation of ultraviolet light. The Mo(CO)_x species can release further CO ligands to form Mo adatoms with CO desorption at 285 K. In addition, a fraction of the released CO ligands transfers onto the Pt surface and subsequently desorbs at 350-550 K. The resulting Mo layer deposited on the Pt surface is nearly free of contamination by C and O. The deposited Mo adatoms can diffuse into the bulk Pt at temperatures above 1070 K.     

BaO/W(1 0 0) thermionic emitters and the effects of Sc, Y, La, and the density functional used in computations

Density functional theory is used to predict workfunctions, φ. For relaxed clean W(1 0 0), the local density approximation (LDA) agrees with experiment better than the newer generalized gradient approximation, probably due to the surface electron self-energy. The large Ba metallic radius indicates it covers W(1 0 0) at about 0.5 monolayer (ML). However, Ba²⁺, O²⁻, and metallic W all have similar radii. Thus 1 ML of BaO (one BaO unit for each two W atoms) produces minimum strain, indicating commensurate interfaces. BaO (1 ML) and Ba (1/2 ML) have the sameφ to within 0.02 V, so at these coverages reduction or oxidation is not important. Due to greater chemical activity of ScO vs. highly ionic BaO, when mixing the latter with this suboxide of scandia, the overlayer always has BaO as the top layer and ScO as the second layer. The BaO/ScO bilayer has a rocksalt structure, suggesting high stability. In the series BaO/ScO/, BaO/YO/, and BaO/LaO/W(1 0 0), the latter has a remarkably low φ of 1.3 V (LDA), but 2 ML of rocksalt BaO also has φ at 1.3 V. We suggest BaO (1 ML) does not exist and that it is worthwhile to attempt the direct synthesis and study of BaO (2 ML) and BaO/LaO.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Fundamental aspects of NOx adsorption on BaO

The nature of NO₂ and nitrite/nitrate pairs adsorbed on BaO has been studied within the density functional theory. Size effects are investigated by comparing results for the BaO molecule, BaO clusters [(BaO)_x, x = 4, 6, 9, 12] and BaO(1 0 0). The adsorption energies show weak dependence on size, which is a manifestation of the fast size convergence of the BaO electronic structure and the local character of the NO₂ adsorption bond. Nitrite/nitrate pair formation is associated with a significant energy gain. For BaO(1 0 0), the stability of the pair is insensitive to the nitrite-nitrate separation, a finding that demonstrates a surface mediated non-local mechanism of molecular pair formation on oxide surfaces. The results have implications for the understanding of NO_x storage and reduction catalysts.     

Characterization of cubic phase MgZnO/Si(1 0 0) interfaces

The microstructural properties of the Mg_xZn_1−xO/Si(1 0 0) interface were investigated using transmission electron microscopy (TEM) and chemical states of the heterostructure were studied by high resolution X-ray photoelectron spectroscopy (XPS). By analyzing the valence band spectra of thin Mg_xZn_1−xO/Si(1 0 0) heterostructures, the valence band offset between such Mg_0.55Zn_0.45O and Si(1 0 0) was obtained to be 2.3 eV. Using the cubic ternary thin films as insulators, metal-insulator-semiconductor (MIS) capacitors have been fabricated. Leakage current density lower than 3 × 10⁻⁷ A/cm² is obtained under the electrical field of 600 kV/cm by current-voltage (I-V) measurement. Frenkel-Poole conduction mechanism is the main cause of current leakage under high electrical field.     

Controlling metal/oxide interactions in bifunctional nanostructured model catalysts: Pd and BaO on Al2O3/NiAl(1 1 0)

We have investigated the structure and growth of Pd and BaO containing nanoparticles sequentially co-deposited on an ordered Al₂O₃/NiAl(1 1 0) by scanning tunneling microscopy (STM), and their interaction with CO and NO₂ by infrared reflection absorption spectroscopy (IRAS).Ba deposition and subsequent oxidation result in Ba_xAl_2yO_x+3y nanoparticles being formed which are characterized by high particle densities and nucleation both on regular terraces and at oxide defects. In contrast, Pd interaction with the model support is weaker and preferential nucleation occurs mainly at rotational domain boundaries and to a lesser extent at anti-phase domain boundaries. For subsequent co-deposition of Pd on preformed Ba_xAl_2yO_x+3y/Al₂O₃/NiAl(1 1 0), Pd nucleates at the Ba_xAl_2yO_x+3y nanoparticles and covers them. The reverse deposition sequence, i.e. subsequent Ba co-deposition and oxidation on preformed Pd/Al₂O₃/NiAl(1 1 0), leads to formation of small isolated Ba_xAl_2yO_x+3y nanoparticles without contact to Pd, together with large Pd crystallites modified by Ba_xAl_2yO_x+3y. The present results provide control over the degree of interaction between metal nanoparticles and oxide nanoparticles on a well-defined model catalyst and thus allow us to study effects related effects on the reactivity and catalytic behavior.     

Formation and hydrogenation of p(2 × 2)-N on Pt(1 1 1)

The formation of a well-ordered p(2 × 2) overlayer of atomic nitrogen on the Pt(1 1 1) surface and its reaction with hydrogen were characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 × 2)-N overlayer is formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to 400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic, rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indicated by the N₂ TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with RAIRS by an intense and narrow (FWHM ∼ 4 cm⁻¹) peak at 3322 cm⁻¹. The areas of the H₂ TPD peak associated with NH dissociation and the XPS N 1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the methods used here.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

RAIRS studies of CO adsorption on Pd/CeO2−x(1 1 1)/Pt(1 1 1)

Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on CeO_2−x-supported Pd nanoparticles at room temperature. The results show that when CeO_2−x is initially grown on Pt(1 1 1), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto CeO_2−x/Pt(1 1 1), most of the Pd grows directly on top of the CeO_2−x(1 1 1). RAIR spectra of CO adsorption on 1 ML Pd/CeO_2−x/Pt(1 1 1) show a broad CO-Pd band, which is inconsistent with a single crystal Pd surface. However, the 5 ML and 10 ML Pd/CeO_2−x/Pt(1 1 1) spectra show vibrational bands consistent with the presence of Pd(1 1 1) and (1 0 0) faces, suggesting the growth of Pd nanostructures with well defined facets.     

The dehydrogenation of CH4 on Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) surfaces: A density functional theory study

CH₄ dehydrogenation on Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) surfaces has been investigated by using density functional theory (DFT) slab calculations. On the basis of energy analysis, the preferred adsorption sites of CH_x (x = 0-4) and H species on Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) surfaces are located, respectively. Then, the stable co-adsorption configurations of CH_x (x = 0-3) and H are obtained. Further, the kinetic results of CH₄ dehydrogenation show that on Rh(1 1 1) and Rh(1 0 0) surfaces, CH is the most abundant species for CH₄ dissociation; on Rh(1 1 0) surface, CH₂ is the most abundant species, our results suggest that Rh catalyst can resist the carbon deposition in the CH₄ dehydrogenation. Finally, results of thermodynamic and kinetic show that CH₄ dehydrogenation on Rh(1 0 0) surface is the most preferable reaction pathway in comparison with that on Rh(1 1 1) and Rh(1 1 0) surfaces.     

Formation of vinylidene and vinylene by selective reactivity of Si(1 0 0)2 × 1 towards iso, cis and trans isomers of dichloroethylene

The room temperature (RT) chemisorption of three (iso, cis and trans) isomers of dichloroethylene (DCE) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 × 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene (m/z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the CCl₂ group favours the diradical dechlorination mechanism. The single m/z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(1 0 0) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(1 0 0)2 × 1. The selective reactivity of the 2 × 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.     

Reduction of FeO/Pt(1 1 1) thin films by exposure to atomic hydrogen

Using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and density functional theory (DFT) calculations we have studied the reduction of ultra-thin films of FeO(1 1 1) grown on Pt(1 1 1) after exposure to atomic hydrogen at room temperature. A number of new ordered, partly-reduced FeO_x structures are identified and as a general trend we reveal that all the reduced FeO_x structures incorporate 2-fold coordinated Fe atoms as opposed to the original 3-fold coordinated Fe atoms in the FeO film. We find that when all the Fe atoms are 2-fold O-coordinated the FeO_x surface structure is resistant to further reduction at room temperature. We observe that water easily dissociates on the most heavily reduced FeO_x, structure in contrast to the initially inert FeO film, and reveal that it is possible to partially re-oxidize the FeO_x film by heating the surface slightly in the presence of water.     

The magnetic map of NiMn alloy thin films on Co(0 0 1) and Co(1 1 1)

Following the experimental work of Groudeva-Zotova et al. [S. Groudeva-Zotova, D. Elefant, R. Kaltofen, D. Tietjen, J. Thomas, V. Hoffmann, C.M. Schneider, J. Magn. Magn. Mater. 263 (2003) 57] where the magnetic and structural characteristics of a bi-layer NiMn-Co exchange biasing systems was investigated, density functional calculations with generalized gradient corrections were performed on (Mn_0.5Ni_0.5)_n ordered alloy on Co(0 0 1) and one Mn_1−xNi_x monolayer on Co(1 1 1). For the Mn_0.5Ni_0.5 monolayer on Co(0 0 1), magnetic moments per surface atom of 0.65 μ_B and 3.76 μ_B were obtained for Ni and Mn, respectively. Those magnetic moments are aligned parallel to the total moment of Co(0 0 1). A complex behavior of the Mn moment in dependence of the thickness “n” is obtained for (Mn_0.5Ni_0.5)_n on Co(0 0 1). Investigations on Mn_1−xNi_x monolayer on Co(1 1 1) have shown that the crystallographic orientation does not modify significantly neither the magnetic moments of Mn and Ni atoms nor their ferromagnetic coupling with the Co(1 1 1) substrate, except for x = 0.66. For x = 0.66 the Mn sub-lattice presents an antiferromagnetic coupling leading to a quenching of the Ni magnetic moment.     

Adsorption of CO, CO2 and NO molecules on a BaTiO3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO₂ and NO molecules on a BaTiO₃ surface following a first preliminary calculation of O and O₂ adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO₂-termination. Consequently, we extended our study to CO, CO₂ and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO₂ molecule directly located above an O surface atom (called O_s) is of the order of −0.18 eV. The O-C distance length is then 1.24 Å and the O-C-O and O-C-O_s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N-O bond is broken when a NO molecule is absorbed on a Ba-O_s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O-Ba-O_s and N-O_s-Ba angles are 106.5° and 63.0°, respectively. The N-O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O_s atom, the N-O axis being tilted toward the Ba atom. The N-O_s-Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.     

Impact of O2 exposure on surface crystallinity of clean and Ba terminated Ge(1 0 0) surfaces

In analogy with the case of Sr on Si [Y. Liang, S. Gan, M. Engelhard, Appl. Phys. Lett. 79 (2001) 3591], we studied surface crystallinity and oxidation behaviour of clean and Ba terminated Ge(1 0 0) surfaces as a function of oxygen pressure and temperature. The structural and chemical changes in the Ge surface layer were monitored by LEED, XPS and real-time RHEED. In contrast to the oxidation retarding effect, observed for 1/2 monolayer of Sr on Si, the presence of a Ba termination layer leads to a pronounced increase in Ge oxidation rate with respect to clean Ge. In fact, while the Ge(1 0 0) surface terminated with 1/2 ML Ba amorphizes for a pO₂ of 10⁻² Torr, LEED indicates that clean Ge forms a thin (4.5 Å), 1 × 1 ordered oxide upon aggressive O₂ exposure (150 Torr, 200 °C, 30 min). We briefly discuss the origins for the difference in behaviour between Ba on Ge and Sr on Si.     

Reaction of water with Ce-Au(1 1 1) and CeOx/Au(1 1 1) surfaces: Photoemission and STM studies

This article reports photoemission and STM studies for the adsorption and dissociation of water on Ce-Au(1 1 1) alloys and CeO_x/Au(1 1 1) surfaces. In general, the adsorption of water at 300 K on disordered Ce-Au(1 1 1) alloys led to O-H bond breaking and the formation of Ce(OH)_x species. Heating to 500-600 K induced the decomposition or disproportionation of the adsorbed OH groups, with the evolution of H₂ and H₂O into gas phase and the formation of Ce₂O₃ islands on the gold substrate. The intrinsic Ce ↔ H₂O interactions were explored by depositing Ce atoms on water multilayers supported on Au(1 1 1). After adsorbing Ce on ice layers at 100 K, the admetal was oxidized immediately to yield Ce³⁺. Heating to room temperature produced finger-like islands of Ce(OH)_x on the gold substrate. The hydroxyl groups dissociated upon additional heating to 500-600 K, leaving Ce₂O₃ particles over the surface. On these systems, water was not able to fully oxidize Ce into CeO₂ under UHV conditions. A complete Ce₂O₃ → CeO₂ transformation was seen upon reaction with O₂. The particles of CeO₂ dispersed on Au(1 1 1) did not interact with water at 300 K or higher temperatures. In this respect, they exhibited the same reactivity as does a periodic CeO₂(1 1 1) surface. On the other hand, the Ce₂O₃/Au(1 1 1) and CeO_2−x/Au(1 1 1) surfaces readily dissociated H₂O at 300-500 K. These systems showed an interesting reactivity for H₂O decomposition. Water decomposed into OH groups on Ce₂O₃/Au(1 1 1) or CeO_2−x/Au(1 1 1) without completely oxidizing Ce³⁺ into Ce⁴⁺. Annealing over 500 K removed the hydroxyl groups leaving behind CeO_2−x/Au(1 1 1) surfaces. In other words, the activity of CeO_x/Au(1 1 1) for water dissociation can be easily recovered. The behavior of gold-ceria catalysts during the water-gas shift reaction is discussed in light of these results.     

The catalytic reduction of NO by H2 on Ru(0 0 0 1): Observation of NHads species

Adsorption of NO and the reaction between NO and H₂ were investigated on the Ru(0 0 0 1) surface by X-ray photoelectron spectroscopy (XPS). Surface composition was measured after NO adsorption and after the selective catalytic reduction of nitric oxide with hydrogen in steady-state conditions at 320 K and 390 K in a 30:1 mixture of H₂ and NO (total pressure = 10⁻⁴ mbar). After steady-state NO reduction, molecularly adsorbed NO in both the linear on-top and threefold coordinations, NH_ads and N_ads species were identified by XPS. The coverage of the NH_ads and N_ads species was higher after the reaction at 390 K than the corresponding values at 320 K. Strong destabilisation of N_ads by O_ads was detected. A possible reaction mechanism is discussed.     

High resolution analysis of the rotational levels of the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states of SO2

A high resolution (0.0018 cm⁻¹) Fourier transform instrument has been used to record the spectrum of an enriched ³⁴S (95.3%) sample of sulfur dioxide. A thorough analysis of the ν₂, 2ν₂ − ν₂, ν₁, ν₁ + ν₂ − ν₂, ν₃, ν₂ + ν₃ − ν₂, ν₁ + ν₂ and ν₂ + ν₃ bands has been carried out leading to a large set of assigned lines. From these lines ground state combination differences were obtained and fit together with the existing microwave, millimeter, and terahertz rotational lines. An improved set of ground state rotational constants were obtained. Next, the upper state rotational levels were fit. For the (0 1 0), (1 1 0) and (0 1 1) states, a simple Watson-type Hamiltonian sufficed. However, it was necessary to include explicitly interacting terms in the Hamiltonian matrix in order to fit the rotational levels of the (0 2 0), (1 0 0) and (1 0 1) states to within their experimental accuracy. More explicitly, it was necessary to use a ΔK = 2 term to model the Fermi interaction between the (0 2 0) and (1 0 0) levels and a ΔK = 3 term to model the Coriolis interaction between the (1 0 0) and (0 0 1) levels. Precise Hamiltonian constants were derived for the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states.     

The plasma oxidation of Pd(1 0 0)

The oxidation of Pd(1 0 0) by an oxygen plasma was characterized using X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (ISS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). The oxygen uptake followed a typical parabolic profile with oxygen coverages reaching 32 ML after 1 h in the plasma; a factor of 40 higher than could be achieved by dosing molecular oxidants in ultra high vacuum. Even after adsorbing 32 ML of oxygen, XPS revealed both metallic Pd and PdO in the surface region. The R27^o LEED pattern previously attributed to a surface oxide monolayer, slowly attenuated with oxygen coverage indicating that the PdO formed poorly ordered three dimensional clusters that slowly covered the ordered surface oxide. While XPS revealed the formation of bulk PdO, only small changes in the ISS spectra were observed once the surface oxide layer was completed. The leading edges of the O₂ TPD curves showed only small shifts with increasing oxygen coverage that could be explained in terms of the lower thermodynamic stability of small oxide clusters. The desorption curves, however, could not be adequately described as simple zero order decomposition of PdO. There has been an ongoing debate in the literature about the relative catalytic activities of PdO and oxygen phases on Pd, the results indicate that any differences in the reactivity between bulk PdO and surface oxides are not associated with differences in the density of exposed Pd atoms or the decomposition kinetics of these two phases.