Size-dependent oxidation of Pdn (n ? 13) on alumina/NiAl(110): Correlation with Pd core level binding energies

Small Pd clusters Pd_n (n = 1, 4, 7, 10, 13) deposited on alumina/NiAl(110) at room temperature were examined by X-ray photoelectron spectroscopy (XPS), as-deposited and after exposure to O₂ at temperatures ranging from 100 to 500 K. After O₂ exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd₄, which did not oxidize under any conditions. The inertness of Pd₄/alumina/NiAl(110) appears to be correlated with a significantly higher-than-expected Pd 3d binding energy, which we attribute to a particularly stable valence shell. None of the clusters examined oxidized during O₂ exposures at 300 K or above, but He⁺ scattering showed that oxygen was bound on the cluster surfaces. Upon heating, all the oxygen associated with these small clusters appeared to spill over and react with the alumina/NiAl(110) support.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.     

In situ XPS study of Pd(1 1 1) oxidation at elevated pressure, Part 2: Palladium oxidation in the 10 mbar range

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 0.4 mbar O₂. The in situ XPS data were complemented by ex situ TPD results. A number of oxygen species and oxidation states of palladium were observed in situ and ex situ. At 430 K, the Pd(1 1 1) surface was covered by a 2D oxide and by a supersaturated O_ads layer. The supersaturated O_ads layer transforms into the Pd₅O₄ phase upon heating and disappears completely at approximately 470 K. Simultaneously, small clusters of PdO, PdO seeds, are formed. Above 655 K, the bulk PdO phase appears and this phase decomposes completely at 815 K. Decomposition of the bulk oxide is followed by oxygen dissolution in the near-surface region and in the bulk. The oxygen species dissolved in the bulk is more favoured at high temperatures because oxygen cannot accumulate in the near-surface region and diffusion shifts the equilibrium towards the bulk species. The saturation of the bulk “reservoir” with oxygen leads to increasing the uptake of the near-surface region species. Surprisingly, the bulk PdO phase does not form during cooling in 0.4 mbar O₂, but the Pd₅O₄ phase appears below 745 K. This is proposed to be due to a kinetic limitation of PdO formation because at high temperature the rate of PdO seed formation is compatible with the rate of decomposition.     

Interaction of Pd cluster anions (Pdn -, n < 11) with oxygen

Interactions between oxygen and Pd cluster anions were studied using Time-of-Flight (ToF) mass spectrometry and Ultraviolet Photoelectron Spectroscopy (UPS). In contrast to the coinage metal clusters, no pronounced size selectivity towards chemisorption of oxygen molecules can be observed for the Pd cluster anions: regardless of the cluster size, no more than 2 oxygen molecules can be attached to a cluster. When Pd_nO_m ^- clusters are prepared by pre-dissociation of oxygen molecules by electric arc and post-reaction with Pd, the proportion of Pd cluster anions reacted with oxygen does not much change compared to the case of the reaction between Pd cluster anions and O₂ molecules. This result indicates that the O₂ chemisorption on Pd cluster anions does not involve large activations barriers: using different synthesis method of a cluster, one can get a better insight into the chemisorption routes of molecules on metal clusters.     

X-ray photoelectron spectroscopy study of Pd oxidation by RF discharge in oxygen

The low-temperature oxidation of polycrystalline palladium by RF oxygen plasma was studied via X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Detailed information about the electronic states of palladium and oxygen was obtained based on the XPS curve fitting analysis of Pd3d and Pd3p + O1s lines. The results showed that Pd oxidation by oxygen plasma was different from Pd oxidation in pure O₂ at high temperature. SEM shows well-structured submicron PdO particles result from oxidation in pure O₂, whereas plasma oxidation results in the predominant formation of two-dimensional PdO structures covering the initial crystallites of the Pd foil. Further oxidation to a three-dimensional PdO phase occurs under prolonged treatment with oxygen plasma. The formation of a PdO_x (x > 1) species, characterized by a E_b(Pd3d_5/2) = 338.0–338.2 eV value that is close to the Pd⁴⁺ oxidation state, was also observed. This PdO_x species was found to have low thermal stability (T < 400 K). It is proposed that the PdO_x species can be localized within the boundaries of crystallites.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

Geometries,stabilities and electronic properties of small-sized Pd2-doped Sin (n = 1–11) clusters

The geometries, growth patterns, relative stabilities and electronic properties of small-sized Pd₂Si_n and Si_n+2 (n = 1–11) clusters are systematically studied using the hybrid density functional theory method B3LYP. The optimised structures revealed that the lowest energy Pd₂Si_n clusters are not similar to those of pure Si_n clusters. When n = 9, one Pd atom in Pd₂Si₉ completely falls into the centre of the Si outer frame, forming metal-encapsulated Si cages. On the basis of the optimised structures, the averaged binding energy, fragmentation energy, second-order energy difference and highest occupied–lowest unoccupied molecular orbital energy gap are calculated. It is found that the Pd₂Si₅ and Pd₂Si₇ clusters have stronger relative stabilities among the Pd₂Si_n clusters. Additionally, the stabilities of Si_n+2 clusters have been reduced by the doping of Pd impurity. The natural population and natural electronic configuration analysis indicated that the Pd atoms possess negative charges for n = 1–11 and there exist the spd hybridisation in the Pd atom. Finally, the chemical hardness, chemical potential, electrostatic potential and polarisability are discussed.     

Pd adsorption on Si(1 1 3) surface: STM and XPS study

Pd-induced surface structures on Si(1 1 3) have been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In the initial process of the Pd adsorption below 0.10 ML, Pd silicide (Pd₂Si) clusters are observed to form randomly on the surface. By increasing the Pd coverage to 0.10 ML, the clusters cover the entire surface, and an amorphous layer is formed. After annealing the Si(1 1 3)-Pd surface at 600 °C, various types of islands and chain protrusions appears. The agglomeration, coalescence and crystallization of these islands are observed by using high temperature (HT-) STM. It is also found by XPS that the islands correspond to Pd₂Si structure. On the basis of these results, evolution of Pd-induced structures at high temperatures is in detail discussed.     

Carbonate formation on Al2O3 thin film model catalyst supports

The formation of carbonate species on alumina upon CO and CO₂ exposure was studied by PM-IRAS and XPS, utilizing the model system of an ultrathin alumina film grown on NiAl(110). No carbonates were detected under UHV conditions, even after exposures up to 100.000 L of the gasses. In contrast, in a 100 mbar CO₂ atmosphere the formation of monodentate carbonates was identified. The surface concentration of the carbonates increased after generating defects on the alumina film by Ar⁺ ion bombardment. It is suggested that this kind of carbonate species is produced by reaction of coordinatively unsaturated O^2? ions of alumina with the C-atom of the CO₂ molecule. This is corroborated by the observation that the amount of carbonates further increased when CO₂ and oxygen were dosed simultaneously. In agreement with the “water gas shift” mechanism previously proposed for carbonate formation on high surface area alumina powders, no carbonates were detected from CO even upon mbar exposure, consistent with the absence of the required OH-groups on the model alumina support.     

Formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys

The formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys were investigated by X-ray photoelectron spectroscopy (XPS) and by adsorption of CO probe molecules, which was characterized by temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The PdAg/Pd(1 1 1) surface alloys were prepared by annealing (partly) Ag film covered Pd(1 1 1) surfaces, where the Ag films were deposited at room temperature. Surface alloy formation leads to a modification of the electronic properties, evidenced by core-level shifts (CLSs) of both the Pd(3d) and Ag(3d) signal, with the extent of the CLSs depending on both initial Ag coverage and annealing temperature. The role of Ag pre-coverage and annealing temperature on surface alloy formation is elucidated. For a monolayer Ag covered Pd(1 1 1) surface, surface alloy formation starts at ∼450 K, and the resulting surface alloy is stable upon annealing at temperatures between 600 and 800 K. CO TPD and HREELS measurements demonstrate that at 120 K CO is exclusively adsorbed on Pd surface atoms/Pd sites of the bimetallic surfaces, and that the CO adsorption behavior is dominated by geometric ensemble effects, with adsorption on threefold hollow Pd₃ sites being more stable than on Pd₂ bridge sites and finally Pd₁ a-top sites.     

Crystal structure and magnetic properties of RCu5−xPdx (R=Pr, Nd, Sm and Eu) alloys

We report the effect of replacing Cu by Pd in RCu₅ (R=Pr, Nd, Sm and Eu). The parent RCu₅ compounds crystallize in the hexagonal CaCu₅-type structure. The hexagonal symmetry is retained in PrCu₄Pd and EuCu_5−xPd_x (x=1 and 2) but the crystal structure changes to cubic AuBe₅-type in PrCu₃Pd₂, NdCu_5−xPd_x (x=1 and 2) and SmCu₄Pd. Substitution with Pd leads to lattice expansion and modifies the magnetic behavior. While PrCu₅ is known to be a van-Vleck paramagnet with a singlet ground state, PrCu₄Pd and PrCu₃Pd₂ show ferromagnetic-like behavior at low temperatures. SmCu₄Pd orders ferromagnetically near 28 K in contrast to the antiferromagnetic nature of the parent SmCu₅. The divalent nature of the Eu ions in EuCu₅ is retained in the ternary alloys, but the Curie temperature is reduced from 57 to 24.5 and 14.5 K in EuCu₄Pd and EuCu₃Pd₂, respectively, inferred from the location of peak in the heat capacity of these two compounds. The magnetic hyperfine field at the Eu nucleus measured with ¹⁵¹Eu Mössbauer spectroscopy in the ternary Eu-alloys is comparable to that in EuCu₅. The magnetic behavior of NdCu₄Pd is similar to that reported in NdCu₅. The zero-field-cooled, low-field magnetization of NdCu₃Pd₂ shows a region of diamagnetic behavior roughly between 21 and 4 K, but the field-cooled response is positive.     

Cluster size effects on hydrazine decomposition on Irn/Al2O3/NiAl(1 1 0)

A series of planar model catalysts were prepared by deposition of size-selected on Al₂O₃/NiAl(1 1 0), and hydrazine decomposition chemistry was used to probe their size-dependent chemical properties. Small Ir_n (n ? 15) on Al₂O₃/NiAl(1 1 0) are able to induce hydrazine decomposition at temperatures well below room temperature, with significant activity first appearing at Ir₇. Both activity and product branching are strongly dependent on deposited cluster size, with these small clusters supporting only the simplest decomposition mechanism: dehydrogenation and N₂ desorption at low temperatures, followed by H₂ recombinative desorption at temperatures above 300 K. For Ir₁₅, we begin to see ammonia production, signaling the onset of a transition to clusters able to support more complex chemistry.     

Scanning tunneling microscopy study of growth of Pt nanoclusters on thin film Al2O3/NiAl(1 0 0)

The growth of Pt nanoclusters on thin film Al₂O₃ grown on NiAl(1 0 0) was studied by using scanning tunneling microscopy (STM). The samples were prepared by vapor depositing various amounts of Pt onto the Al₂O₃/NiAl(1 0 0) at different substrate temperatures in ultra high vacuum (UHV). The STM images show that sizeable Pt nanoclusters grow solely on crystalline Al₂O₃ surface. These Pt clusters appear to be randomly distributed and only a few form evident alignment patterns, contrasting with Co clusters that are highly aligned on the crystalline Al₂O₃. The size distributions of these Pt clusters are rather broader than those of the Co clusters on the same surface and the sizes are evidently smaller. With increasing coverage or deposition temperature, the number of larger clusters is enhanced, while the size of the majority number of the clusters remains around the same (0.4 nm as height and 2.25 nm as diameter), which differs drastically from the Pt clusters on γ-Al₂O₃/NiAl(1 1 0) observed earlier. These Pt cluster growth features are mostly attributed to smaller diffusion length and ease to form stable nucleus, arising from strong Pt-Pt and Pt-oxide interactions and the peculiar protrusion structures on the ordered Al₂O₃/NiAl(1 0 0). The thermal stability of Pt nanoclusters was also examined. The cluster density decreased monotonically with annealing temperature up to 1000 K at the expense of smaller clusters but coalescence is not observed.     

Adsorption of methanol on Ni3Al(1 1 1) and NiAl(1 1 0): A high resolution PES study

The adsorption of methanol on Ni₃Al(1 1 1) and NiAl(1 1 0) has been studied using high resolution photoemission spectroscopy (HR-PES) and density functional theory (DFT). Both methanol and methoxy are formed on these surfaces after the initial methanol exposure at low temperatures. Heating to 200 K leads to further formation of methoxy. On NiAl(1 1 0) two different methoxy species are observed where the first is formed upon methanol adsorption, and the other results from methanol decomposition during heating. The DFT calculations show that methanol and methoxy interacts with the Al atoms on both surfaces. Methanol is found to bond through the oxygen atom to the Al on-top site on Ni₃Al(1 1 1) and NiAl(1 1 0) with the C–O axis tilted with respect to the surface normal. On Ni₃Al(1 1 1) methoxy is situated in a 2Ni+Al hollow site, whereas on NiAl(1 1 0) the Al–Al bridge site is preferred.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.     

Growth and decay of the Pd(1 1 1)-Pd5O4 surface oxide: Pressure-dependent kinetics and structural aspects

Growth and decomposition of the Pd₅O₄ surface oxide on Pd(1 1 1) were studied at sample temperatures between 573 and 683 K and O₂ gas pressures between 10⁻⁷ and 6 × 10⁻⁵ mbar, by means of an effusive O₂ beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd₅O₄ surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10⁻⁶ mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O₂ pressure pulse and then following further growth kinetics in the lower (10⁻⁶ mbar) pressure range.Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd₅O₄ layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a superstructure.Isothermal decay of the Pd₅O₄ oxide layer at 693 K involves at first a rearrangement into the structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.     

Free radical reactions at the Ru(0 0 0 1) surface: Atomic oxygen and dissociated NH3

X-ray photoelectron spectroscopy was used to study the effect of atomic oxygen on Ru(0 0 0 1), and the effect of dissociated ammonia on RuO₂/Ru(0 0 0 1), in UHV conditions at ambient temperature. The Ru(0 0 0 1) surface was exposed, at ambient temperature, to a mixed flux of atomic and molecular oxygen generated by dissociation of O₂ in a thermal catalytic cracker, with 45% dissociation efficiency. The detailed study of the XPS spectra shows the formation of a disordered multilayer oxide (RuO₂). No formation of higher oxides of Ru was observed. The formation of RuO₂ proceeded without saturation for total oxygen exposures of up to 10⁵ Langmuir, at which point an average oxide thickness of 68 Å was observed. RuO₂ formed by the reaction with atomic oxygen was exposed to a flux of NH_x (x = 1, 2) + H generated by the cracker. The reduction of RuO₂ to Ru metal was observed by XPS. An exposure of 3.6 × 10² L of NH_x + H, resulted in the observation of adsorbed H₂O and OH, but no evidence of lattice oxide. The chemisorbed species were removed by additional NH_x + H exposure. No nitrogen adsorption was observed.     

Cluster Structure of Liquid Alcohols,Water, and <Emphasis Type="Italic">n</Emphasis>-Hexane

The processes of cluster formation in liquid alcohols, water, methanol, n-hexanol, and n-hexane have been investigated by the method of flicker-noise spectroscopy. Two types of clusters — clusters with a close-packed structure and clusters with a loose structure — have been detected. The energy of formation of different clusters in methanol and n-hexane ranges, respectively, from −250 to +250 J/mole and from −50 to +50 J/mole. The smallest clusters of methanol, n-hexanol, water, and n-hexane consist, respectively, of six, two, eleven, and two molecules, and their largest clusters represent oscillators consisting, respectively, of 50,400, 17,200, 93,500, and 33,150 molecules at 274 K. In methanol at 271 K, more than 44 types of clusters consisting of 6, 97, 152, 219, 297, 492, 1029, 1368, 1560, etc. molecules were detected. In n-hexanol at 273 K, 57 types of clusters were detected. Models of small clusters are proposed. In water, the content of close-packed clusters is maximum at 277 K. The energy of formation/decomposition of small clusters in water ranges from −0.4 to +0.4 kJ/mole and increases with increase in the water temperature. The hysteresis of transformation of the (H₂O)₂₈₀ cluster in the process of heating and cooling of water in the temperature range 273–280 K was detected. Series of energy spectra of clusters in liquids at different temperatures are presented and discussed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 305–312, May–June, 2005.     

A density functional theory study of small bimetallic PdnAl (n=1–8) clusters

The geometries, stabilities, and magnetic properties of Pd_nAl (n=1–8) neutral clusters are studied using density functional theory with generalized gradient approximation. The growth pattern for different sized Pd_nAl (n=1–8) clusters is Al-substituted Pd_n+1 clusters and it keeps the similar framework of the most stable Pd_n+1 clusters except n=6 and 8. Al atoms in the ground state Pd_nAl isomers tend to occupy the most highly coordinated position. The analysis of stabilities shows that doping an Al atom can enhance the stabilities of the host Pd clusters and the magic number characteristic of Pd₄ cluster cannot be changed, the Pd₃Al cluster has a higher stability. Charges are transferred from Al atom to Pd atoms in all Pd_nAl clusters, so the Al atom is the electron donor, and Pd atoms are the electron accepters. Doping an Al atom decreases the average atomic magnetic moments of the host Pd clusters.     

Enhanced activity for supported Au clusters: Methanol oxidation on Au/TiO2(110)

Gold clusters supported on TiO₂(110) exhibit unusual activity for the oxidation of methanol to formaldehyde. Temperature programmed desorption studies of methanol on Au clusters show that both Au and titania sites are necessary for methanol reaction. Isotopic labeling experiments with CD₃OH demonstrate that reaction occurs via OH bond scission to form a methoxy intermediate. When the TiO₂ surface is oxidized with ¹⁸O₂ before or after Au deposition, methanol reaction produces H₂¹⁸O below 300 K, indicating that oxygen from titania promotes OH bond scission and is incorporated into desorbing products. XPS experiments provide additional evidence that during methanol reaction on the Au/TiO₂ surface, methanol adsorption occurs on TiO₂, given that the titania support becomes slightly oxidized after exposure to methanol in the presence of Au clusters. While the role of TiO₂ is to dissociate the OH bond and form the reactive methoxy intermediate, the role of the Au sites is to remove hydrogen from the surface as H₂, thus preventing the recombination of methoxy and hydrogen to methanol. The decrease in formaldehyde yield with increasing Au coverage above 0.25 ML suggests that reaction occurs at Au–titania interfacial sites; scanning tunneling microscopy images of various Au coverages confirm that the number of interfacial sites at the perimeter of the Au clusters decreases as the Au coverage is increased between 0.25 and 5 ML.