Pd 3d Core Level Research Articles

Hydrogen absorption and diffusion behaviors in cube-shaped palladium nanoparticles revealed by ambient-pressure X-ray photoelectron spectroscopy

The hydrogen absorption and diffusion processes in cube-shaped palladium (Pd) nanoparticles (NPs) were studied by the combination of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations. A size dependence of the subsurface hydrogen absorption was observed. More hydrogen atoms were absorbed in the subsurface of the smaller-sized Pd NPs owing to the enhancement of the diffusivity of H atoms from the surface into the subsurface rather than the adsorption or absorption rates at the initial stages. This results from the weakened Pd-H bond caused by surface disordering of Pd NPs with the reduction of the size of the particles. Furthermore, we discuss the H absorption sites in the bulk by comparing the relative Pd 3d core-level binding energy shifts of the Pd atoms obtained from the AP-XPS results and the theoretical calculations. The octahedral (O) sites are shown to be more favorable than the tetrahedral (T) sites for hydrogen occupation by comparing the experimental results and theoretical calculations. Finally, we proposed an interaction model between hydrogen and the Pd NPs during H2 absorption and diffusion to provide new insights into the hydrogen absorption process.

High rates of catalytic hydrogen combustion with air over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ Ti 0.97 Pd 0.03 O 2 - δ coated cordierite monolith

$$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta }$$ was coated on $$\upgamma \hbox {-}\hbox {Al}_{2}\hbox {O}_{3}$$ -coated honeycomb structured cordierite monolith (AHCM) by solution combustion method using dip-dry-heat process. This is a modified conventional method to coat the catalysts on honeycomb structured cordierite monolith (HCM). Formation of $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ on AHCM was confirmed by XRD. The XPS spectra of Pd(3d) core level confirmed Pd ion in $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ is in the form of +2 state. The surface morphology of coated catalyst was unchanged by long time exposure to hydrogen combustion reaction; i.e., $$\hbox {H}_{2} + \hbox {O}_{2}$$ recombination reaction which indicated the stability of coating on monolith. $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ showed high rates of $$\hbox {H}_{2} + \hbox {O}_{2}$$ recombination compared to 2 atom% Pd(metal)/ $$\upgamma \hbox {-}\hbox {Al}_{2}\hbox {O}_{3}$$ , $$\hbox {Ce}_{0.98}\hbox {Pd}_{0.02}\hbox {O}_{2-\updelta } $$ , $$\hbox {Ce}_{0.98}\hbox {Pt}_{0.02}\hbox {O}_{2-\updelta } $$ , $$\hbox {Ce}_{0.73}\hbox {Zr}_{0.25}\hbox {Pd}_{0.02}\hbox {O}_{2-\updelta }$$ , $$\hbox {Ti}_{0.99}\hbox {Pd}_{0.01}\hbox {O}_{2-\updelta } $$ and $$\hbox {Ti}_{0.98}\hbox {Pd}_{0.02}\hbox {O}_{2-\updelta } $$ . The activation energy of $$\hbox {H}_{2} + \hbox {O}_{2}$$ recombination reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ is 7.8 kcal/mol. The rates of reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ at 60 $$^{\circ }\hbox {C}$$ are in the range between 10 and 20 $$\upmu \hbox {mol/g/s}$$ . The rate of reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ increased with increase in the concentration of $$\hbox {H}_{2}$$ . For 50 mL of $$\hbox {H}_{2}$$ , it showed rates of the reaction around 36.45 $$\upmu \hbox {mol/g/s}$$ at room temperature and 230 $$\upmu \hbox {mol/g/s}$$ at 60 $$^{\circ }\hbox {C}$$ . It was found that the rate of reaction due was lower due to hindering effect by adsorption of other gas molecules on the catalytic site. Finally, we propose a mechanism of hydrogen and oxygen recombination reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ . $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ catalysts were coated by solution combustion method on $$\upgamma \hbox {-}\hbox {Al}_{2}\hbox {O}_{3}$$ coated honeycomb structured cordierite monolith. Among the catalysts, $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ coated monolith showed high catalytic activity for hydrogen combustion with air. The proposed mechanism of $$\hbox {H}_{2} + \hbox {O}_{2}$$ reaction clarified that $$\hbox {Pd}^{+2}$$ ion state is unaltered in $$\hbox {Ti}\hbox {O}_{2}$$ lattice.

Nature of the N–Pd Interaction in Nitrogen-Doped Carbon Nanotube Catalysts

In this work, the geometric and electronic structure of N species in N-doped carbon nanotubes (NCNTs) is derived by X-ray photoemission (XPS) and absorption spectroscopy (NEXAFS) of the N 1s core excitation. Substitutional N species in pyridine-like configuration and another form of N with higher thermal stability are found in NCNTs. The structural configuration of the high thermally stable N species, in the literature often referred to as graphitic N, is assessed in this work by a combined theoretical and experimental study as a 3-fold substitutional N species in an NCNT basic structural unit (BSU). Furthermore, the nature of the interaction of those N species with a Pd metal center immobilized onto NCNTs is of σ-type donation from the filled π-orbital of the N atom to the empty d-orbital of the Pd atom and a π back-donation from the filled Pd atomic d-orbital to the π* antibonding orbital of the N atom. We have found that the interaction of pyridine N with Pd is characterized by a charge transfer typical of a covalent chemical bond with partial ionic character, consistent with the chemical shift observed in the Pd 3d core level of divalent Pd. Graphitic N sites interact with Pd by a covalent bond without any charge redistribution. In this case, the electronic state of the Pd corresponds to metallic Pd nanoparticles electronically modified by the interaction with the support. The catalytic reactivity of these samples in hydrogenation, CO oxidation, and oxygen reduction reaction (ORR) allowed clarifying some aspects of the metal carbon support interaction in catalysis.

Insights into the effects of surface properties of oxides on the catalytic activity of Pd for C-C coupling reactions.

Understanding the interaction between Pd nanocatalysts and metal oxide supports for heterogeneous C-C coupling reactions is still ambiguous since many factors influence the catalytic behavior of Pd nanocatalysts. Herein, three porous nanorods of CeO2 with controllable surface properties were employed as supports for Pd nanocatalysts with similar dispersion, which avoided the impact of other factors including surface area, morphology and accessible active sites. It provides an ideal approach to probe synergetic catalytic behavior of metal nanoparticles on supports. The results obtained by studying three C-C coupling reactions (Ullman, Suzuki and Heck) indicate a strong correlation between the surface properties of supports and the catalytic activity of Pd nanocatalysts: supports with a strong basicity and a high concentration of oxygen vacancies result in a rich electron density of Pd and accelerate the first step of oxidative addition reaction for C-C coupling. The infrared spectroscopic study on ν[CO] of CO-treated catalysts and XPS analysis of the Pd(3d) core level provide strong evidence supporting the interaction of Pd/supports for C-C coupling reactions.

Adsorption of Au and Pd on Ruthenium-Supported Bilayer Silica

Adsorption of Au and Pd over bilayer SiO2/Ru has been investigated using scanning-probe microscopy, X-ray photoemission spectroscopy (XPS), and density functional theory (DFT). Low temperature (∼5 K) atomic force (AFM) and scanning tunneling microscopy (STM) measurements reveal the presence of small adsorption features after exposing the samples to small doses of either metal. In the case of Pd, we note a homogeneous distribution of adsorbates across the entire surface, which consists of both amorphous and crystalline phases. Au, however, adsorbs only over amorphous areas and domain boundaries, which possess larger pores than can be found in the ordered portions of the film. DFT calculations reveal that this difference is rooted in the pore-size-dependent barriers for diffusion of the two metals into the film, where they can then bind stably at the Ru interface. Auger parameter analysis of the Pd 3d and Au 4f core-levels from atoms binding in this manner show upward orbital-energy shifts, which, according...

Surface composition of clean and oxidized Pd75Ag25(100) from photoelectron spectroscopy and density functional theory calculations

High resolution photoelectron spectroscopy and density functional theory calculations have been used to study the composition of clean and oxidized Pd75Ag25(100). The results for the clean surface confirm earlier reports of surface segregation by Wouda et al. (1998), where the top most layers are rich in Ag. The Pd 3d core level component from the surface region is observed at higher binding energies than the contribution from the bulk which is found to be a signature of Pd embedded in Ag. Low energy electron diffraction and scanning tunneling microscopy measurements reveal that oxidation of the Pd75Ag25(100) surface results in a 5×5R27°-O structure similar to the one reported for Pd(100). The calculations suggest that the stable structure is a PdO(101) monolayer supported on a (100) surface rich in Ag at the interface to the stoichiometric alloy. The calculated core level shifts for the oxidized surface are in good agreement with the experimental observations.

Temperature-Induced Valence Transition of EuPd2Si2 Studied by Hard X-ray Photoelectron Spectroscopy

We have studied the electronic structure of EuPd2Si2 by hard X-ray photoelectron spectroscopy (HX-PES) from 300 to 20 K. The temperature-dependent HX-PES spectra clearly show the valence transition, namely, the intensities of the divalent and trivalent Eu 3d components are abruptly changed. The change in Eu 3d spectral shape, especially the drastic change in the trivalent Eu 3d feature with temperature, can be explained within the framework of the Anderson model. The peak shift of Pd 3d core level with temperature indicates that the valence electrons of Pd contribute to the temperature-induced valence transition of EuPd2Si2.

Determining the Chemical Reactivity Trends of Pd/Ru(0001) Pseudomorphic Overlayers: Core-Level Shift Measurements and DFT Calculations

We have addressed the problem of determining a reliable experimental descriptor of surface chemical reactivity by measuring Pd and Ru 3d(5/2) core-level shifts of the Pd-n/Ru(0001) pseudomorphic overlayer system (n = 0-3) by high resolution X-ray photoelectron spectroscopy. We find a linear relationship between the calculated theoretical changes of the d-band center position projected on each Ru and Pd atomic layer (which is, according to the Hammer and Norskov d-band model, a good theoretical descriptor of chemical reactivity) and the corresponding core-level shifts, both for the Ru atomic planes and for the Pd overlayers. Core-level shifts, therefore, should be considered as reliable experimental descriptors of chemical reactivity in the same sense (and with similar limitations) of the theoretical descriptor d-band center. Final-state effect contributions to the shifts do not obscure this trend.

Comparison of the interaction of Pd with positively and negatively poled LiNbO 3(0 0 0 1)

To investigate the possibility of manipulating the surface chemical properties of finely dispersed metal films through ferroelectric polarization, the interaction of palladium with oppositely poled LiNbO 3(0 0 0 1) substrates was characterized. Low energy ion scattering indicated that the Pd tended to form three-dimensional clusters on both positively and negatively poled substrates even at the lowest coverages. X-ray photoelectron spectroscopy (XPS) showed an upward shift in the binding energy of the Pd 3d core levels of 0.9 eV at the lowest Pd coverages, which slowly decayed toward the bulk value with increasing Pd coverage. These shifts were independent of the poling direction of the substrate and similar to those attributed to cluster size effects on inert supports. Thus, the spectroscopic data suggested that Pd does not interact strongly with LiNbO 3 surfaces. The surface chemical properties of the Pd clusters were investigated using CO temperature programmed desorption. On both positively and negatively poled substrates, CO desorption from freshly deposited Pd showed a splitting of the broad 460 K desorption peak characteristic of bulk Pd into distinct peaks at 270 and 490 K as the Pd coverage was decreased below 1.0 ML; behavior that also resembles that seen on inert supports. It was found that a small fraction of the adsorbed CO may dissociate (<2%) for Pd on both positively and negatively poled substrates. The thermal response of the smaller Pd clusters on the LiNbO 3 surfaces, however, was different from that of inert substrates. In a manner similar to Nb 2O 5, when CO desorption experiments were carried out a second time, the adsorption capacity decreased and the higher temperature desorption peak shifted from 490 K to below 450 K. This behavior was independent of the substrate poling direction. Thus, while there was evidence that LiNbO 3 does not behave as a completely inert support, no significant differences between positively and negatively poled surfaces were observed. This lack of sensitivity of the surface properties of the Pd to the poling direction of the substrate is attributed to the three-dimensional Pd clusters being too thick for their surfaces to be influenced by the polarization of the underlying substrate.

Electronic structure of PdAg(1 0 0) ordered surface alloys using synchrotron radiation

PdAg(1 0 0) ordered surface alloys have been prepared by e-beam evaporation technique in situ in UHV and were investigated using high energy photoemission and normal incidence X-ray standing wave (NIXSW) techniques with wiggler radiation. The line shapes of 3d core levels of Ag and Pd exhibited interesting changes with the increase of Pd concentration in the alloys. The Ag 3d core level exhibited a disorder induced broadening that gradually increased with Pd concentration. On the other hand Pd 3d core level exhibited an asymmetry on the high binding energy side of the peak which gradually increased with Pd concentration. Interestingly, Pd core level did not exhibit a broadening with increase of Pd content in the alloy.

Low pressure oxidation of ordered Sn/Pd(110) surface alloys

The reaction of oxygen at low pressure with the Sn/Pd(110) system has beenexamined by photoelectron spectroscopy using synchrotron radiation. Thec(2 × 2) and(3 × 1) reconstructions of the Sn/Pd(110) surface at 0.5 and 0.7 monolayers(ML) Sn coverage and a 1.75 ML Sn overlayer on the Pd(110) surface afterflashing to 470 K were studied. The Sn 4d core level is strongly affected byO2 adsorption while the Pd 3d core level shows very little change other than a decreasein intensity. Starting with a 10 L dose of oxygen, prominent changes in thespectra were observed for all Sn/Pd(110) surface alloys. Analysis of the Sn 4dcore levels indicates that oxidation proceeds with the formation of well-definedstates of Sn, which were identified as a Pd–Sn–O interface layer, SnO andSnO2 oxides. The valence band spectra confirm this assignment. TheSn2+ andSn4+ component signals originate from the topmost surface layer, i.e. tin atoms in more highlyoxidized states constitute the topmost surface layer on top of the Pd–Sn–O interface.The presence of a sub-surface PdSn intermetallic alloy facilitates the tin oxideformation; the Sn–O phase formation is accompanied by Pd–Sn bond dissociation.

Electronic structure of Pd nanoparticles on carbon nanotubes

The effect of the oxygen plasma treatment on the electronic states of multi-wall carbon nanotubes (MWCNTs) is analyzed by X-ray photoemission measurements (XPS) and UPS, both using synchrotron radiation. It is found that the plasma treatment effectively grafts oxygen at the CNT-surface. Thereafter, the interaction between evaporated Pd and pristine or oxygen plasma-treated MWCNTs is investigated. Pd is found to nucleate at defective sites, whether initially present or introduced by oxygen plasma treatment. The plasma treatment induced a uniform dispersion of Pd clusters at the CNT-surface. The absence of additional features in the Pd 3d and C 1s core levels spectra testifies that no Pd–C bond is formed. The shift of the Pd 3d core level towards high-binding energy for the smallest clusters is attributed to the Coulomb energy of the charged final state.

Core and valence level photoemission and photoabsorption study of icosahedral Al–Pd–Mnquasicrystals

The electronic structure of quasicrystalline Al–Pd–Mn is investigated by means of valenceand core level photoelectron spectroscopy. Variations of the photoionization cross section inthe constituents’ valence electronic levels as a function of photon energy are used toidentify contributions from the different atomic species, in particular near the Pd 4d Cooperminimum. Resonant photoemission at the Mn 2p absorption edge shows the contribution ofthe Mn 3d states to the density of states in a region near the Fermi level. Theasymmetry of Pd 3d and Mn 2p core level photoemission lines, and its differencefor emission from metallic and quasicrystalline phases, are utilized to infer thecontributions of the different constituents to the density of states at the Fermi level.

Electronic properties of Sn/Pd intermetallic compounds on Pd(1 1 0)

We have studied the Sn/Pd(1 1 0) adsorption system by synchrotron radiation photoelectron spectroscopy and low-energy electron diffraction (LEED). For room temperature evaporation, two surface reconstructions were observed: c(2 × 2) and (3 × 1), corresponding to about 0.5 ML and 0.75 ML of Sn adlayer coverage. The Pd 3d and Sn 4d core levels as well as valence band spectra indicate a strong chemical interaction between Sn and Pd, and the formation of an intermetallic interface. Structural models are proposed for both of these phases based on the photoemission and CO adsorption results. We show that at coverage higher than 0.7 ML, tin is alloyed with the Pd crystal forming a subsurface layer of Pd–Sn intermetallic compound of stoichiometry which varies with tin coverage. CO adsorption occurs only at low temperature (120 K) and depends on the Sn coverage and reconstruction of the Pd(1 1 0) surface. We estimate the CO adsorption energy for the c(2 × 2)- and (3 × 1)-Sn/Pd(1 1 0) surfaces to be reduced by 40% compared to the clean palladium (1 1 0) surface.

Changes in hydrogen permeability and surface state of Pd–Ag/ceramic composite membranes after thermal treatment

Changes in hydrogen permeability and surface state of a Pd–Ag/ceramic membrane have been investigated as a function of thermal treatment. The surface state is determined by X-ray photoelectron spectroscopy (XPS). The XPS data show a significant enrichment of the Ag alloying element and an accumulation of impurity atoms (C, S, Na, Cl, O) in the surface layer of an as-prepared membrane. The thermal treatment, conducted at 400 °C in air for 1 h, results in a decrease of the Pd/Ag ratio in the surface (from 1:3.4 to 1:2.9), and an obvious chemical shift (ca. 1.4 eV) of Pd 3d core level but no change in the Ag 3d level, indicating the Pd 2+ state of PdO is formed. The thermal treatment does not change the surface states of impurities of Na, Cl and O. On the other hand, XPS signal of the oxidized sulfur, SO 4 2− species, is found. It is noted that the carbon concentration is decreased dramatically (from 67 to 32 at.%) by decomposition of carbon, a new XPS peak (BE = 292.5 eV) of CO 2 is clearly identified from a treated surface. After the thermal treatment, the hydrogen permeability of the membrane is increased by a factor of 2.5, such behavior can be mainly attributed to the removing of the carbon contaminate initially present in the surface.

XPS, TDS and static SIMS studies of binary Pd/Al system properties: correlation between Pd–Al bimetallic interaction and CO adsorption

Carbon monoxide (CO) adsorption properties of Pd/Al systems have been studied using X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS) and static secondary ion mass spectroscopy (SSIMS) under UHV conditions. Measured XPS and TDS spectra were compared to that of clean bulk metals (Pd, Al). Pd adlayers, prepared by vacuum evaporation onto a pure polycrystalline Al surface, exhibited a strong interaction with Al atoms resulting in formation of a Pd–Al alloy of a noble metal-like electronic structure. The shifts of the Pd 3d core levels and the Pd 4d-band centroid toward higher binding energies due to bimetallic bond formation were observed. The CO TDS spectra consisted of two desorption peaks, both lying lower than the one typical for a Pd foil, indicating distinct weakening of the Pd–CO chemisorption bond. Partial CO dissociation on the Pd/Al surface during adsorption/desorption cycles, accompanied by CO 2 and H 2O production, was observed. CO dissociation caused creation of Pd–C and Al–O chemical bonds on the Pd/Al surface resulted in weakening of the Pd–Al bimetallic bond.

Electronic structure of UxPd100$minus;x films prepared by sputter co-deposition

We studied, by photoelectron spectroscopy (UPS, XPS), the transition of the U 5f states from localised behaviour in UPd3 and more diluted alloys, to itinerant behaviour in U metal, through UxPd100−x films prepared by sputter co-deposition. The U 4f core level peak evolves from a single poorly screened peak (at high dilutions, [U]/[Pd]<0.25) to a single well-screened peak in U metal. In a transition domain ([U]/[Pd]≅0.3–0.5) the two peaks coexist. The Pd 3d core level was shown to shift to high binding energy (BE) with increasing U concentration. This shift is discussed in terms of U–Pd bonding and changing hybridisation of the Pd 4d states in the alloys. Valence band spectra show a shift of the Pd 4d band to high BE and its narrowing as concentration increases. In UPd3, the density of states (DOS) at the Fermi-level is low, and the U 5f states are withdrawn to high BE. When the U concentration increases, itinerant U 5f states appear right at the Fermi-level.

Co-adsorption of CO and Pb on Pd [formula omitted

The adsorption of Pb on Pd (1 1 0) and co-adsorption with CO have been studied by photoemission spectroscopy and low energy electron diffraction. Two ordered structures are formed at sub-monolayer coverages of Pb with symmetry c(2×2) and (3×1). The valence band and Pd 3d core level spectra indicate a strong chemical interaction between Pb and Pd, and the formation of intermetallic bonds with both the first and second layer of Pd. The heat of CO adsorption on the c(2×2) phase is reduced to about half of the value on clean Pd, and even less on the (3×1) structure so that CO does not adsorb at a temperature of 120 K. Structural models are proposed for both of these phases based on the photoemission and co-adsorption results. The results confirm that the de-activation by Pb of Pd as a CO oxidation catalyst is due to the formation of an intermetallic compound, which drastically lowers the adsorption energy. However even the formation of a sub-monolayer phase is sufficient to radically alter the catalytic properties, and the formation of a bulk intermetallic is not necessary.

Surface alloying of Pd and Cu on Ru(0 0 0 1)

Codeposition of Pd and Cu on Ru(0001) with total coverages varying from submonolayer to nm thickness has been carried out at temperatures of 300 and 550 K. The effects of heating the overlayers up to 1250 K were studied. Besides the characterization of the overlayer composition as a function of temperature and observation of possible ordering, a series of questions is addressed. This includes conditions for alloying, possible effects related to dimensionality (2- or 3-dimensions), and the significance of the order of deposition. Photoelectron spectroscopy (PES) based on application of synchrotron radiation and the Mg Kα X-rays (XPS) was used to obtain information on the electronic structure of the bimetallic overlayers via the Ru 3d, Pd 3d core levels (PES) and Cu 2p core levels (XPS). Furthermore, the valence band (VB) electron energy distribution curves (EDC's) were measured at a photon energy of 40 eV which is the Cooper minimum of Cu 3d electrons. LEED was used to establish the order of the overlayers.The main findings of the bimetallic studies are that (i) the deposited Cux and Pd1−x for 0⩽x⩽1 with a total coverage of 1 ML, are distributed in one layer with temperature dependent intermixing, (ii) the valence bands are very characteristic with the Cu 3d band separated from the Pd 4d band and alloying gives rise to new features with binding energies (BE's) between these bands, (iii) the Cu 2p core level shifts (CLS) are correlated linearly with the composition of the films in 2- and 3-dimensions, an effect which is attributed to the different coordination numbers between Pd and Cu in the different dimensions, (iv) the structure of the overlayers depends only slightly on the order of deposition of the two coadsorbates for T>660 K and (v) (1×1) LEED patterns are obtained for 1 ML Cu+2 ML Pd at room temperature and for 2 ML Cu+2 ML Pd after annealing at 660 K indicating, in combination with the pronounced hybridization of Cu 3d and Pd 4d orbitals, formation of a surface alloy.

Co-adsorption of CO and Pb on Pd(1 1 0)

The adsorption of Pb on Pd(110) and co-adsorption with CO have been studied by photoemission spectroscopy and low energy electron diffraction. Two ordered structures are formed at sub-monolayer coverages of Pb with symmetry c(2×2) and (3×1). The valence band and Pd 3d core level spectra indicate a strong chemical interaction between Pb and Pd, and the formation of intermetallic bonds with both the first and second layer of Pd. The heat of CO adsorption on the c(2×2) phase is reduced to about half of the value on clean Pd, and even less on the (3×1) structure so that CO does not adsorb at a temperature of 120 K. Structural models are proposed for both of these phases based on the photoemission and co-adsorption results. The results confirm that the de-activation by Pb of Pd as a CO oxidation catalyst is due to the formation of an intermetallic compound, which drastically lowers the adsorption energy. However even the formation of a sub-monolayer phase is sufficient to radically alter the catalytic properties, and the formation of a bulk intermetallic is not necessary.