An FTIR study of the bonding of methoxy on Ni(100): effects of coadsorbed sulfur, carbon monoxide and hydrogen

The vibrational spectra of CH₃O_(a), CD₃O_(a), CDH₂O_(a) and CD₂HO_(a) on Ni(100) are analyzed and interpreted in terms of resonances between fundamental modes and either combinations or overtones. Analysis of the symmetry of the modes observed suggests that methoxy binds normal to the surface with C_s symmetry, at least at low coverages. Two distinct vibrational bands emerge in the vibrational spectrum of methoxy in the v(CO) region as the coverage increases which are attributed to bonding in four-fold hollow sites and bridging sites. These bands exhibit blue shifts of about 25 cm⁻¹ with increasing coverage up to the saturation coverage. The vibrational bands in the v(CH) region appear concomitantly at all coverages and shift down 12 cm⁻¹ as the coverage is increased. These shifts are attributed to changes in the metal-oxygen bond which are reflected in changes in the strength of the C---O and C---H bonds. Affects on the bonding also appear to occur with the coadsorption of hydrogen or CO with methoxy. Coadsorption of 0.36 ML hydrogen with 0.04 ML methoxy induces blue shifts of 15 and 7 cm⁻¹ for the v(CO) bands at 949 and 984 cm⁻¹, respectively. Adsorbing 0.43 ML of CO with 0.04 ML methoxy (and 0.04 ML hydrogen) causes a red shift of 20 and 12 cm⁻¹ for these bands. A drastic drop in mode intensities for methoxy when CO is coadsorbed suggests that the methoxy tilts away from the surface normal. Pre-adsorbing sulfur on the Ni(100) surface reduces the amount of methoxy formed from methanol, but the v(CO) methoxy bands are unshifted in frequencies relative to their position for the same methoxy coverage on the clean surface.     

Observation of true c(8 × 2) symmetry in scanning tunnelling microscopy images of the clean InSb(001) surface

Filled and empty state scanning tunnelling microscopy images of the sputtered and annealed InSb(001) surface are presented. The sputter-anneal preparation generates a surface with two distinct phases. The dominant phase possesses a unit cell with true c(8 × 2) symmetry, whereas the other phase is attributed to an asymmetric 1 × 3 reconstruction. The presence of a c(8 × 2) unit cell in filled state images is in contrast to previous reports, which identified only a 4 × 1 unit cell. The true c(8 × 2) symmetry further indicates, the available structural model is used as a guide, that the current interpretation of features in filled state images is incorrect. This result may necessitate a reevaluation of the structural model for the InSb(001)-c(8 × 2) surface.     

Density functional theory analysis of the Ni(1 1 0)c(2 × 2)-CN surface phase

Density functional theory slab calculations have been used to investigate the structure of the Ni(1 1 0)c(2 × 2)-CN adsorption phase. The results show excellent agreement with experimental quantitative determinations of this structure by photoelectron diffraction and low energy electron diffraction. In particular, they show that a lying-down orientation with the C–N axis along [0 0 1] perpendicular to the close-packed Ni rows on the surface is strongly favoured over end-on adsorption (with the C–N axis perpendicular to the surface). This geometry is also favoured over a lying-down geometry with the C–N axis aligned along the azimuth, as originally proposed for this system and supported by cluster calculations.     

STM study of low pressure adsorption of silane on Si(111)7 × 7

We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH₂ species by chemisorption and involves the breaking of Si---Si backbonds of the adatom.     

Chemisorption of isolated Br atoms on Si(100)-(2 × 1) studied by PAC

Chemisorption and desorption of isolated bromine adatoms on the Si(100)-(2 × 1) surface were investigated with nuclear methods. Br adsorption sites at low coverages of 10⁻³ monolayers (ML) were characterised by measuring the nuclear quadrupole interaction with perturbed γγ-angular correlation (PAC) of ⁷⁷Br→⁷⁷Se probe atoms. Below room temperature, two distinct adsorption sites for Br are revealed by PAC. One of them disappears after isochronal annealing above 300 K. The more stable probe-atom state is associated with single Br atoms saturating a dangling bond of the surface, while the less stable state is attributed to adsorption of Br at a bridge site. The potential barrier between the two adsorption sites is estimated to be 0.9(1) eV. At temperatures above 550 K, the fraction of atoms on distinct sites decreases, presumably due to surface diffusion. By measuring the γ-activity of the sample, complete desorption of the ⁷⁷Br atoms was observed above 620 K.     

Pd deposits on Ni(111): a theoretical study

The ASED-MO method has been used to gather electronic and energetic information on Pd deposits on Ni(111) and Pd atom inclusion in the first Ni layer since these model catalysts exhibit a striking catalytic efficiency towards butadiene hydrogenation. The electronic structure of Pd atoms is strongly altered compared with pure Pd. A Pd(4d)→Pd(5s) electronic transfer occurs in the case of the deposit when a slight similar transfer and a charge transfer from Pd to surrounding Ni takes place in the case of the inclusion. Those results are consistent with XPS experimental data. A low density of states, near the Fermi level, is also observed. The optimal geometrical situation for Pd deposits is found to be 2D-aggregates (in pseudoepitaxy or pseudomorphy with the underlying Ni surface, depending on the aggregate size). Small aggregates (part of the first Ni layer) are found to be the most stable in the case of a Pd inclusion in the Ni with a Pd---Pd distance of 2.64 Å, in agreement with available EXAFS experimental data.     

The adsorption of C6H5Cl on Si(111)7 × 7 studied by STM

The adsorption of chlorobenzene on Si(111)7 × 7 at room temperature was studied by scanning tunneling microscopy (STM). Selective chemisorption was observed at different adatom sites. It was found that the center adatoms were more reactive than the corner adatoms, and the faulted half of the unit cell was more reactive than the unfaulted. The mechanism is discussed in terms of the electronic and atomic structures in Si(111)7 × 7. Both preferences indicate that chlorobenzene was present initially in a mobile precursor state.     

Density functional theory investigation of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0)

The adsorption of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(1 1 1) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C-N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(1 1 1); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(1 1 1)/CN and Ni(1 0 0)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(1 1 1) which has proved challenging for DFT calculations. On Ni(1 0 0) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(1 1 0).     

The local structure of SO2 and SO3 on Ni(1 1 1): A scanned-energy mode photoelectron diffraction study

O 1s and S 2p scanned-energy mode photoelectron diffraction (PhD) data, combined with multiple-scattering simulations, have been used to determine the local adsorption geometry of the SO₂ and SO₃ species on a Ni(1 1 1) surface. For SO₂, the application of reasonable constraints on the molecular conformation used in the simulations leads to the conclusion that the molecule is centred over hollow sites on the surface, with the molecular plane essentially parallel to the surface, and with both S and O atoms offset from atop sites by almost the same distance of 0.65 Å. For SO₃, the results are consistent with earlier work which concluded that surface bonding is through the O atoms, with the S atom higher above the surface and the molecular symmetry axis almost perpendicular to the surface. Based on the O 1s PhD data alone, three local adsorption geometries are comparably acceptable, but only one of these is consistent with the results of an earlier normal-incidence X-ray standing wave (NIXSW) study. This optimised structural model differs somewhat from that originally proposed in the NIXSW investigation.     

First-principles study of oxidized Nb(1 0 0) surface structures

The atomic positions of the oxygen-induced c(2 × 2)-O, (3 × 1)-O and (4 × 1)-O surface structures on Nb(1 0 0) are determined by first-principles electronic structure calculations within the density functional theory comparing experimentally observed scanning tunneling microscopy (STM) images. STM images of these surfaces are calculated on the basis of the theory of Tersoff and Hamann. The theoretical and experimental STM images of the oxygen-chemisorbed c(2 × 2)-O structural model agree well. However, only the oxide-covered (3 × 1)-O and (4 × 1)-O structural models with two layers of NbO and contraction of the unit length along longitudinal 〈1 0 0〉 direction by 10% result in the theoretical STM images that agree with the experimental ones.     

A theoretical investigation of adsorbate-induced surface relaxation effects using cluster models: Al on Si(111)

Differently sized cluster calculations are used to investigate theoretically the preferred adsorption site for an Al adatom on the Si(111) surface. By performing partial geometry optimizations at the Hartree-Fock level on AlSi_n subclusters around the site of interest we find significant Al adatom-induced surface relaxation effects distorting the Si atoms from their bulk lattice positions. The largest relaxation effects take place at the T₄ site resulting in Al adsorption at the T₄ site to be 5 kcal/mol more stable than at the H₃ site and considerably more stable than adsorption at the T₁ site. However, we only have confidence in this result after performing for the T₄ site a partial geometry optimization on the AlSi₅ subcluster in the AlSi₂₆H₂₄ cluster and by including appropriate correlation corrections.     

The formation and characterisation of Ni — an X-ray photoelectron spectroscopic investigation of potassium-doped Ni(110)–O

The interaction of potassium with an oxidised Ni(110) single-crystal surface has been investigated using X-ray photoelectron spectroscopy (XPS). Incorporation of the alkali metal at room temperature results in suppression of the Ni(2p) satellite structure of NiO and is explained on the basis of a recent theoretical model. Unambiguous evidence is provided for the formation of Ni³⁺ after annealing this surface, and at 600 K it is the majority species in the surface region.     

Observation of new two-dimensional periodic structures in the confined regions of the reconstructed Pt(100) surface

New 2D periodic structures have been observed by STM in the regions of the reconstructed Pt(100) surface which are confined by domain boundaries or lattice steps. These structures can be seen only in a very narrow energy window near the Fermi level, and they are strongly correlated to the original atomic arrangements of the surface. These structures arise most probably from a modification in the distribution of the electronic density of states which is strongly coupled to the ion cores of the surface to produce a periodic shift in the atomic rearrangements in order to minimize the strain and free energy of the surface.     

A reversible reaction forming (---Cu---O---) strings and (Cu)6-clusters on Ag(110) shown by STM

An artificial new surface of (---Cu---O---) chains grown on Ag(110) surface was prepared by reacting a surface with Cu atoms, where the (---Cu---O---) chains grow in the [1 0] direction and are self-assembled on the Ag(110) surface in a (2 x 2)-p2mg structure. When the Cu---O/Ag(110) surface was heated in vacuum, the (---Cu---O---) chain decomposed to uniform cluster dots arranged along the [1 0] direction, where the cluster dots were composed of six Cu atoms. When the Ag(110) surface with the Cu---cluster dots was exposed to O₂, the (---Cu---O---) lines were redrawn along the [1 0] direction by reacting a s in the [1 0] direction with O₂. This is a reversible chemical reaction in one dimensional regime proved in atomic resolution.     

STM/nc-AFM investigation of (n×6) reconstructed GaAs(0 0 1) surface

We have investigated the n×6 reconstructed GaAs(0 0 1) surface with scanning tunneling and non-contact atomic force microscopy techniques (STM/nc-AFM). For the first time atomically resolved nc-AFM images of that surface are shown. The images confirm the presence of rows of arsenic dimers in the topmost layer as predicted by the current model of n×6 reconstructed surface. However, in contrast to previous reports we found that postulated As dimer sites are not fully occupied. Moreover, the images suggest that ×6 symmetry is present on the surface even in absence of the dimers. We show that due to probing of different surface properties nc-AFM and STM are complementary tools for complex surfaces investigation.     

The adsorption and decomposition of NO on Pd(110)

The sticking probability of nitric oxide (NO) on Pd(110) and the relative selectivity of the surface to nitrogen (N₂) and nitrous oxide (N₂O) production has been measured as a function of coverage and as a function of surface and gas temperatures using a molecular beam. It is found that, at low temperatures (<440 K), molecular adsorption occurs with an initial sticking probability of 0.40 ± 0.02, rising quickly to a maximum of about 0.48 ± 0.02 as coverage increases before falling towards saturation. Following adsorption at 170 K four distinct adsorption sites can be identified by subsequent TPD. Hence, if beaming occurs at a temperature above the TPD peak due to a given site, then that site cannot be populated and the saturation coverage is found to be reduced. At higher temperatures (440–650 K) the sticking probability is seen to decrease continuously as a function of coverage. At a given NO uptake, the sticking probability falls with temperature indicating that the rate of NO desorption is significant in this temperature range. In addition, dissociation occurs leading to the desorption of nitrogen and nitrous oxide leaving only oxygen adatoms on the surface. The oxygen adatoms poison further reaction but can be cleaned off, even at the lowest temperature at which dissociation occurs, by hydrogen or carbon monoxide. At the low temperature end of this range more nitrous oxide is produced than nitrogen but this ratio falls with temperature until, above 600 K, there is 100% selectivity to the production of nitrogen which we propose is due to the low lifetime of molecular NO on the surface. However, at such high temperatures, reaction only occurs on a few sites probably located at the few step edges present on the crystal.     

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.     

Molecular beam studies of ethanol oxidation on Pd(110)

The adsorption and decomposition of ethanol on Pd(110) has been studied by use of a molecular beam reactor and temperature programmed desorption. It is found that the major pathway for ethanol decomposition occurs via a surface ethoxy to a methyl group, carbon monoxide and hydrogen adatoms. The methyl groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decompose (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temperature (450 K) hydrogen peak is observed in TPD, resulting from the decomposition of stable hydrocarbon fragments. A competing pathway also exists which involves C---O bond scission of the ethoxy, probably caused by a critical ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decomposition pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide respectively. Below 250 K, some ethanol can form acetate which decomposes around 400 K to produce carbon dioxide and hydrogen.     

A surface X-ray diffraction study of TiO2(1 1 0)(3 × 1)-S

Surface X-ray diffraction has been used to investigate the structure of TiO₂(1 1 0)(3 × 1)-S. In concert with existing STM and photoemission data it is shown that on formation of a (3 × 1)-S overlayer, sulphur adsorbs in a position bridging 6-fold titanium atoms, and all bridging oxygens are lost. Sulphur adsorption gives rise to significant restructuring of the substrate, detected as deep as the fourth layer of the selvedge. The replacement of a bridging oxygen atom with sulphur gives rise to a significant motion of 6-fold co-ordinated titanium atoms away from the adsorbate, along with a concomitant rumpling of the second substrate layer.