Adsorption and decomposition of NO on K-deposited Pd(1 1 1)

The adsorption and decomposition of NO on a K-deposited Pd(1 1 1) surface were investigated using X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and temperature-programmed desorption. For the K-deposited Pd(1 1 1) surface, two different NO adsorption sites were observed in addition to the Pd site. On the clean Pd(1 1 1) surface, the adsorption of NO was purely molecular and reversible, but on the K-deposited surface, the adsorbed NO decomposed at two different temperatures, 530 and 610 K. These results indicate that the NO adsorption and decomposition sites were newly created by the deposition of K onto the Pd(1 1 1) surface.     

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.     

Electronic properties of thin Zn layers on Pd(1 1 1) during growth and alloying

The growth and alloying of thin Zn layers on Pd(1 1 1) was investigated using X-ray and ultraviolet photoelectron spectroscopy as well as low energy electron diffraction and correlated with density functional calculations. At 105 K, the formation of a pseudomorphic Zn monolayer is observed. Upon heating this layer to 550 K or upon deposition of 1 ML at 550 K, an ordered p(2 × 1) PdZn surface alloy with a Pd:Zn ratio of ∼1:1 is formed, with a characteristic Pd 3d_5/2 peak at a binding energy of ∼335.6 eV. For deposition of 3 ML Zn at 550 K or by heating 3 ML, deposited at low temperature, to 500 or 600 K, a PdZn alloy with a Pd:Zn ratio of again ∼1:1 is found in the surface region, with a Pd 3d_5/2 peak at ∼335.9 eV; the direct preparation at 550 K leads to a more homogeneous and better ordered alloy. The valence band spectrum of this alloy with a low density of states at the Fermi level and pronounced maxima due to the “Pd 4d” band at ∼2.4 and 3.9 eV closely resembles the spectrum of Cu(1 1 1), in good agreement with the calculated density of states for a PdZn alloy of 1:1 stoichiometry. The shift of the “Pd 4d” band to higher binding energies as compared to Pd(1 1 1) indicates a charge transfer from Zn to the Pd 4d levels. Overall, the similarity between the ultraviolet photoelectron spectra for the PdZn alloy and for Cu(1 1 1) is taken as explanation for the similar chemical activity of both systems in methanol steam reforming.     

Adsorption site, core level shifts and charge transfer on the Pd(1 1 1)-I(√3 × √3) surface

We use core level photoelectron spectroscopy and density functional theory (DFT) to investigate the iodine-induced Pd(1 1 1)-I(√3 × √3) structure formed at 1/3 ML coverage. From the calculations we find that iodine adsorbs preferentially in the fcc hollow site. The calculated equilibrium distance is 2.06 Å and the adsorption energy is 68 kcal/mol, compared to 2.45 Å and 54 kcal/mol in the atop position. The adsorption energy difference between fcc and hcp hollows is 1.7 kcal/mol. Calculated Pd 3d surface core level shift on clean Pd(1 l 1) is 0.30 eV to lower binding energy, in excellent agreement with our experimental findings (0.28-0.29 eV). On the Pd(1 1 1)-I(√3 × √3) we find no Pd 3d surface core level shift, neither experimentally nor theoretically. Calculated charge transfer for the fcc site, determined from the Hirshfeld partitioning method, suggests that the iodine atom remains almost neutral upon adsorption.     

Growth of epitaxial thin Pd(1 1 1) films on Pt(1 1 1) and oxygen-terminated FeO(1 1 1) surfaces

Thin Pd films (1-10 monolayers, ML) were deposited at 35 K on a Pt(1 1 1) single crystal and on an oxygen-terminated FeO(1 1 1) monolayer supported on Pt(1 1 1). Low energy electron diffraction, Auger electron spectroscopy, and Kr and CO temperature programmed desorption techniques were used to investigate the annealing induced changes in the film surface morphology. For growth on Pt(1 1 1), the films order upon annealing to 500 K and form epitaxial Pd(1 1 1). Further annealing above 900 K results in Pd diffusion into the Pt(1 1 1) bulk and Pt-Pd alloy formation. Chemisorption of CO shows that even the first ordered monolayer of Pd on Pt(1 1 1) has adsorption properties identical to bulk Pd(1 1 1). Similar experiments conducted on FeO(1 1 1) indicate that 500 K annealing of a 10 ML thick Pd deposit also yields ordered Pd(1 1 1). In contrast, annealing of 1 and 3 ML thick Pd films did not result in formation of continuous Pd(1 1 1). We speculate that for these thinner films Pd diffuses underneath the FeO(1 1 1).     

Comparative DFT study of the adsorption of 1,3-butadiene, 1-butene and 2-cis/trans-butenes on the Pt(1 1 1) and Pd(1 1 1) surfaces

The interaction of 1,3-butadiene, 1-butene and 2-cis/trans-butenes on the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied with density functional theory methods (DFT). The same most stable adsorption modes have been found on both metal surfaces with similar adsorption energies. For 1,3-butadiene the 1,2,3,4-tetra-σ adsorption structure is shown to be the most stable one, in competition with a 1,4-metallacycle-type mode, which is only less stable by 10-12 kJ mol⁻¹. On Pt(1 1 1) these total energy calculations were combined with simulations of the vibrational spectra. This confirms that the 1,2,3,4-tetra-σ adsorption is the most probable adsorption structure, but cannot exclude the 1,4-metallacycle as a minority species. Although similar in type and energy, the adsorption on the Pd(1 1 1) surface shows a markedly different geometry, with a smaller molecular distortion upon adsorption. The most stable adsorption structure for the butene isomers is the di-σ-mode. Similarly to the case of the 1,3-butadiene, the adsorption geometry is closer to the gas phase one on Pd than on Pt, hence explaining the different spectroscopic results, without the previously assumed requirement of a different binding mode. Moreover the present study has shown that the different selectivity observed on Pt(1 1 1) and Pd(1 1 1) for the hydrogenation reaction of butadiene cannot be satisfactory explained by the single comparison of the relative stabilities of 1,3-butadiene and 1-butene on these metals.     

The adsorption of acetic acid on Au/Pd(1 1 1) alloy surfaces

The adsorption of acetic acid is studied as a function of gold content by temperature-programmed desorption and reflection-absorption infrared spectroscopy on Au/Pd(1 1 1) alloys formed by depositing 5 ML of gold onto a Pd(1 1 1) surface and heating to various temperatures. For mole fractions of gold greater than ∼0.5, acetic acid adsorbs molecularly and desorbs intact with an activation energy of ∼52 kJ/mol. This acetic acid is present as catemers, where the nature of the catemer is found to depend on gold concentration. When the relative gold concentration is less than ∼0.33, adsorption of acetic acid at 80 K and heating to ∼207 K forms η¹-acetate species on the surface. On further heating, these can either thermally decompose to eventually evolve hydrogen, water and oxides of carbon, or form η²-acetate species, where the coverage of reactively formed η²-acetate species increases with decreasing gold concentration in the near surface region.     

Structure and reaction of oxametallacycles derived from styrene oxide on Ag(1 1 0)

Styrene oxide forms a strongly bound oxametallacycle intermediate via activated adsorption on the Ag(1 1 0) surface. The oxametallacycle species derived from styrene oxide on Ag(1 1 0) fits well with the family of oxametallacycles identified previously in studies of non-allylic epoxides with unsaturated substituent groups on silver. Temperature-programmed reaction experiments demonstrate that styrene oxide ring opens at the substituted carbon, and Density Functional Theory calculations indicate that the phenyl ring of the resulting oxametallacycle is oriented nearly parallel to the Ag(1 1 0) surface. Interaction of the phenyl group with the silver surface stabilizes this intermediate relative to that derived from the mono-olefin epoxide, ethylene oxide. During temperature-programmed reaction, the oxametallacycle undergoes ring-closure to reform styrene oxide and isomerization to phenylacetaldehyde at 505 K on Ag(1 1 0). Styrene oxide-derived oxametallacycles exhibit similar ring-closure behavior on the Ag(1 1 1) surface.     

Kinetic Monte Carlo simulations of Pd deposition and island growth on MgO(1 0 0)

The deposition and ripening of Pd atoms on the MgO(1 0 0) surface are modeled using kinetic Monte Carlo simulations. The density of Pd islands is obtained by simulating the deposition of 0.1 ML in 3 min. Two sets of kinetic parameters are tested and compared with experiment over a 200-800 K temperature range. One model is based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer is the only diffusing species. The other is based upon transition rates obtained from density functional theory calculations which show that small Pd clusters are also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations show that both models reproduce the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects is attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the DFT-based model, small clusters form already on terraces and diffuse to defects. In the DFT-based model, the strong dimer and trimer binding energies at charged oxygen vacancy defects prevent island ripening below the experimentally observed onset temperature of 600 K.     

Structural evolution of sulfur overlayers on Pd(1 1 1)

Low energy ion scattering spectroscopy (LEISS) has been used to characterize the evolution of ordered structures of S on the Pd(1 1 1) surface during annealing. During exposure of the Pd(1 1 1) surface to 0.7 L H₂S at 300 K—conditions that produce the S(√3 × √3)R30 overlayer—the intensity of the Pd LEIS signal decreases and a feature assigned to adsorbed S appears as the adsorbed layer forms. When the surface is held at 300 K after exposure to H₂S is stopped, the LEIS Pd intensity partially recovers and the S signal weakens, presumably as surface S atoms assume their equilibrium positions in the S(√3 × √3)R30 overlayer. Subsequent annealing of the S(√3 × √3)R30 structure at 700 K causes it to convert into a S(√7 × √7)R19 overlayer, whose LEIS spectrum is identical to that of clean Pd(1 1 1). The absence of LEIS evidence for S atoms at the exposed surface of the S(√7 × √7)R19 overlayer is at odds with published models of a mixed Pd-S top layer. Despite the similarity of the LEIS spectra of Pd(1 1 1) and Pd(1 1 1)-S(√7 × √7)R19, their activities for dissociative hydrogen adsorption are very different—the former readily adsorbs hydrogen at 100 K, while the latter does not—suggesting that S exerts its influence on surface chemistry from subsurface locations.     

Formation and characterization of Au/Pd surface alloys on Pd(1 1 1)

The formation of alloys by adsorbing gold on a Pd(1 1 1) single crystal substrate and subsequently annealing to various temperatures is studied in an ultrahigh vacuum by means of Auger and X-ray photoelectron spectroscopy. The nature of the alloy surface is probed by CO chemisorption using temperature-programmed desorption and reflection-absorption infrared spectroscopy. It is found that gold grows in a layer-by-layer fashion on Pd(1 1 1) at 300 K, and starts to diffuse into the bulk after annealing to above ∼600 K. Alloy formation results in a ∼0.5 eV binding energy decrease of the Au 4f XPS signals and a binding energy increase of the Pd 3d features of ∼0.8 eV, consistent with results obtained for the bulk alloy. The experimentally measured CO desorption activation energies and vibrational frequencies do not correlate well with the surface sites expected from the bulk alloy composition but are more consistent with significant preferential segregation of gold to the alloy surface.     

Sum frequency generation and density functional studies of CO-H interaction and hydrogen bulk dissolution on Pd(1 1 1)

CO-H interaction and H bulk dissolution on Pd(1 1 1) were studied by sum frequency generation (SFG) vibrational spectroscopy and density functional theory (DFT). The theoretical findings are particularly important to rationalize the experimentally observed mutual site blocking of CO and H and the effect of H dissolution on coadsorbate structures. Dissociative hydrogen adsorption on CO-precovered Pd(1 1 1) is impeded due to an activation barrier of ∼2.5 eV for a CO coverage of 0.75 ML, an effect which is maintained down to 0.33 ML CO. Preadsorbed hydrogen prevented CO adsorption at 100 K, while hydrogen was replaced from the surface by CO above 125 K. The temperature-dependent site blocking of hydrogen originates from the onset of hydrogen diffusion into the Pd bulk around 125 K, as shown by SFG and theoretical calculations using various approaches. When Pd(1 1 1) was exposed to 1:1 CO/H₂ mixtures at 100 K, on-top CO was absent in the SFG spectra although hydrogen occupies only threefold hollow sites on Pd(1 1 1). DFT attributes the absence of on-top CO to H atoms diffusing between hollow sites via bridge sites, thereby destabilizing neighboring on-top CO molecules. According to the calculations, the stretching frequency of bridge-bonded CO with a neighboring bridge-bonded hydrogen atom is redshifted by 16 cm⁻¹ when compared to bridging CO on the clean surface. Implications of the observed effects on hydrogenation reactions are discussed and compared to the C₂H₄-H coadsorption system.     

Ethylene dissociation on flat and stepped Ni(1 1 1): A combined STM and DFT study

The dissociative adsorption of ethylene (C₂H₄) on Ni(1 1 1) was studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM studies reveal that ethylene decomposes exclusively at the step edges at room temperature. However, the step edge sites are poisoned by the reaction products and thus only a small brim of decomposed ethylene is formed. At 500 K decomposition on the (1 1 1) facets leads to a continuous growth of carbidic islands, which nucleate along the step edges.DFT calculations were performed for several intermediate steps in the decomposition of ethylene on both Ni(1 1 1) and the stepped Ni(2 1 1) surface. In general the Ni(2 1 1) surface is found to have a higher reactivity than the Ni(1 1 1) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the different reaction pathways. In particular the barrier for dissociation is lowered significantly more than the barrier for dehydrogenation, and this is of great importance for the bond-breaking selectivity of Ni surfaces.The influence of step edges was also probed by evaporating Ag onto the Ni(1 1 1) surface. STM shows that the room temperature evaporation leads to a step flow growth of Ag islands, and a subsequent annealing at 800 K causes the Ag atoms to completely wet the step edges of Ni(1 1 1). The blocking of the step edges is shown to prevent all decomposition of ethylene at room temperature, whereas the terrace site decomposition at 500 K is confirmed to be unaffected by the Ag atoms.Finally a high surface area NiAg alloy catalyst supported on MgAl₂O₄ was synthesized and tested in flow reactor measurements. The NiAg catalyst has a much lower activity for ethane hydrogenolysis than a similar Ni catalyst, which can be rationalized by the STM and DFT results.     

O2 dissociation on Pd(2 1 1) and Cu(2 1 1) surfaces

We have performed ab initio Density Functional Theory (DFT) based calculations to observe the reactivity of the Pd(2 1 1) and Cu(2 1 1) surfaces towards O₂. In order to properly address the adsorption dynamics, the static potential energy surface calculations have been complemented with first principles molecular dynamics calculations, which reveal interesting steering effects that complicate the dissociation dynamics. We have found that on both surfaces the step microfacets are very reactive and the dissociation of the O₂ molecule at room temperature occurs mostly on those sites.     

Structural study of Co/Pd(1 1 1) and CO/Co/Pd(1 1 1) by surface X-ray absorption fine structure spectroscopy

The structure of the Co thin films on Pd(1 1 1) and the effect of the CO adsorption on Co thin films were studied by Co K-edge surface X-ray absorption fine structure (XAFS). The polarization dependences of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicate that Co thin films grow in the fcc stacking mode on Pd(1 1 1) up to 12 ML. The analysis of the nearest neighbor shell shows little mechanical strain at the interface, indicating that Co atom does not grow pseudomorphically on Pd(1 1 1). There is no alloy-like structure at the interface. CO adsorption causes no structural change of the Co thin films but modifies the Co surface electronic state. These structural studies provide deep insight in the magnetic property of the Co thin films on Pd(1 1 1).     

First-principles calculations of ethanethiol adsorption and decomposition on GaN (0 0 0 1) surface

The adsorption and decomposition of ethanethiol on GaN (0 0 0 1) surface have been investigated with first-principles calculations. The DFT calculations reveal that ethanethiol adsorbs dissociatively on the clean GaN (0 0 0 1) surface to form ethanethiolate and hydrogen species. An up limit coverage of 0.33 for ethanethiolate monolayer on GaN (0 0 0 1) surface is obtained and the position of the sulfur atom and the tilt angle of the thiolate chain are found to be very sensitive to the surface coverage. Furthermore, the reactivity of ethanethiol adsorption and further thermal decomposition reactions on GaN (0 0 0 1) surface is discussed by calculating the possible reaction pathways and ethene is found to be the major product.     

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) surfaces

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) stepped surfaces has been investigated by the extended London-Eyring-Polyani-Sato (LEPS) method constructed using a 5-parameter Morse potential. The calculated results show that there exist common characteristics of CO adsorption on the two surfaces. At low coverage, CO occupies threefold hollow site of the (1 1 1) terrace and is tilted with respect to the surface normal. Among the threefold hollow sites on the (1 1 1) terrace, the nearer the site is to the step, the greater is the influence of the step. The twofold bridge site on the (1 0 0) step is also a stable adsorption site at high coverage. Because of the different lengths of the (1 1 1) terraces, the (3 1 1) and (2 1 1) stepped surfaces have different characteristics. A number of new sites are exposed on the boundary regions, including the fourfold hollow site (H₄) of the (3 1 1) surface and the fivefold hollow site (H₅) of the (2 1 1) surface. At high coverage, CO resides in the H₅ site of the (2 1 1) surface, but the H₄ site of the (3 1 1) surface is not a stable adsorption site. This study further shows that the on-top site on the (1 0 0) step of Pd(3 1 1) is a stable adsorption site, but the same type of site on Pd(2 1 1) is not.     

Chemistry of Alanine on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The adsorption of alanine is studied on a Pd(1 1 1) surface using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is found that alanine adsorbs into the second and subsequent layers prior to completion of the first monolayer for adsorption at ∼250 K, while at ∼300 K, alanine adsorbs almost exclusively into the first monolayer with almost no second-layer adsorption. Alanine adsorbs onto the Pd(1 1 1) surface in its zwitterionic form, while the multilayer contains about 30-35% neutral alanine, depending on coverage. Alanine is thermally stable on the Pd(1 1 1) surface to slightly above room temperature, and decomposes almost exclusively by scission of the CCOO bond to desorb CO₂ and CO from the COO moiety, and the remaining fragment yields ethylamine and HCN.     

Chemisorption of silyl radicals onto Pd(1 0 0) surface: A computational DFT study

We have investigated the adsorption of silane and silyl radicals onto a Pd(1 0 0) surface. The solid metal has been modelled by a 24-atom-2-layer Pd cluster. Density Functional Theory calculations have been performed for the different species approaching the surface, adopting the hybrid B3LYP functional. It is found that all species, in the hollow position, dissociate up to SiH, which is then stable in this position. Dissociated hydrogen atoms are chemisorbed onto the surface in distinct bridge positions. Dissociation mechanism is studied through computation of selected sections of the potential energy surface.     

First-principles calculations of C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0)

Density functional theory calculations are performed to investigate the C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0). The calculated geometric parameters indicate the center of doped Ag is located above the Ni(1 0 0) surface owing to the size mismatch. The C binding on the alloy surface is substantially weakened, arising from the less attractive interaction between C and Ag atoms, while in the subsurface, the C adsorption is promoted as the Ag coverage is increased. The effect of substitutional Ag on the adsorption property of Ni(1 0 0) is rather short-range, which agrees well with the analysis of the projected density of states. Seven pathways are constructed to explore the C diffusion behavior on the bimetallic surface. Along the most kinetically favorable pathway, a C atom hops between two fourfold hollow sites via an adjacent octahedral site in the subsurface of reconstructed Ag/Ni(1 0 0). The “clock” reconstruction which tends to improve the surface mobility, is more favorable on the alloy surface because the c(2 × 2) symmetry is inherently broken by the Ag impurity. As a consequence, the local lattice strain induced by the C transport is effectively relieved by the Ag-enhanced surface mobility and the C diffusion barrier is lowered from 1.16 to 0.76 eV.